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THE  ETHEL  CARR  PEACOCK 

MEMORIAL  COLLECTION 


Matris  amort  monumentum 


TRINITY  COLLEGE  LIBRARY 

DURHAM,  N.  C. 

1903 


Gift  of  Dr.  and  Mrs.  Dred  Peacock 


Digitized  by  the  Internet  Archive 
in  2016  with  funding  from 
Duke  University  Libraries 


https://archive.org/details/elementsofnatura1879houst 


'L\£‘2^ 


THE  ELEMENTS 


OF 

Natural  Philosophy. 


tf{^  of  ^ofjaob  mttl 


BY 

EDWIN  J.  HOUSTON,  A.M., 

PROFESSOR  OF  PHYSICAL  GEOGRAPHY  AND  NATURAL  PHILOSOPHY  IN  THE  CENTRAL 
HIGH  SCHOOL  OF  PHILADELPHIA;  AUTHOR  OF  "ELEMENTS 
OF  PHYSICAL  GEOGRAPHY." 


PHILADELPHIA : 

ELDREDGE  & BROTHER, 

No.  17  North  Seventh  Street. 

1879. 


A SERIES  OF  TEXT-BOOKS 

ON 


THE  NATURAL  SCIENCES. 

By  Prof.  E.  J.  HOUSTON. 


1.  Easy  Lessons  in  Natural  Philosophy. 

2.  Elements  of  Natural  Philosophy. 

3.  Elements  of  Physical  Geography. 


Entered,  according  to  Act  of  Congress,  in  the  year  1879,  by 
ELDREDGE  & BROTHER, 
in  the  OflBce  of  the  Librarian  of  Congress,  at  Washington. 


'T 


J.  FAGAN  i BON 
1^  ELECTROTYPERS,  PHILAP’A. 




S' 30 . 2. 
P 


TN  the  “ Elements  of  Natural  Philosophy  ” will  be  found 
the  more  important  principles  of  the  science.  The  great 
variety  of  subjects  embraced  by  Natural  Philosophy,  makes 
it  impracticable,  in  the  limits  of  an  elementaiy  work,  to 
give  more  than  the  mere  outlines  of  the  science.  These 
the  author  has  endeavored  to  state  in  a concise  form  and  in 
logical  sequence,  so  that  the  book,  though  small,  shall  form 
a simple  system  of  Natural  Philosophy,  and  not  a mere  col- 
lection of  disconnected  facts. 

The  remarkable  progress  made  in  this  department  of 
natural  science  within  the  past  few  years,  has  rendered  it 
advisable,  in  the  opinion  of  the  author,  to  depart  somewhat 
widely  from  the  methods  of  arrangement  usually  adopted 
in  elementary  text-books.  This  is  more  particularly  the 
case  in  the  treatment  of  electricity,  where  magnetism,  in- 
stead of  preceding  the  general  subject,  is  made  to  occupy 
a subordinate  place  as  one  of  the  effects  of  an  electrical 
current.  The  general  division  into  electrical  charge  and 
current  electricity  will  also,  it  is  believed,  aid  the  student 
in  obtaining  clear  conceptions  of  the  principles  of  elec- 
tricity. 

Although  the  importance  of  the  study  of  Natural  Philos- 
ophy is  now  almost  universally  acknowledged,  yet  the  ex- 
pense of  cabinets  of  philosophical  apparatus  is  so  great. 


IV 


PREFACE. 


that  a more  general  introduction  of  the  study  has  been  pre- 
vented. The  author  has  introduced  into  “ The  Elements  of 
Natural  Philosophy,”  a feature  which  he  trusts  will  to  a 
great  extent  remove  this  objection.  Throughout  the  text 
will  be  found  descriptions  of  simple  experiments  that  can 
be  made  with  apparatus  so  easily  contrived,  as  not  only  to 
render  it  possible  for  the  teacher  to  illustrate  the  subject 
inexpensively,  but  also  to  permit  the  experiments  to  he  made 
by  the  students  themselves,  thus  enabling  them  to  acquire 
that  intimate  knowledge  of  the  subject  that  can  come  only 
by  self-conducted  experiments. 

While  the  book  is  thus  suited  to  the  use  of  such  schools 
as  are  \inable  to  provide  costly  cabinets  of  ajsparatus,  the 
standard  pieces  found  in  well  selected  cabinets  are  care- 
fully described  and  figured,  thus  adapting  the  book  to  the 
use  of  schools  well  provided  with  apparatus. 

A carefully  prepared  syllabus,  and  series  of  questions  for 
review,  will  be  found  following  each  chapter. 

An  examination  of  the  book  will  show  that  no  expense 
has  been  spared  to  bring  it  up  to  the  highest  standard  as 
regards  illustrations,  typography,  paper,  and  printing. 

The  author  desires  to  acknowledge  his  indebtedness  to 
his  friend  Prof.  Elihu  Thomson  for  critical  examination  of 
the  proof-sheets. 

E.  J.  H. 


Central  High  School, 
Philadelphia,  May,  1879. 


PART  1. 

MATTER  AND  FORCE. 

CHAPTER  I 

Matter 

Syllabus  

Questions  foe  Review 


CHAPTER  II. 

General  Properties  of  Matter 16 

Syllabus 26 

Questions  for  Review 27 

CHAPTER  III. 

The  Three  Conditions  of  Matter .29 

Syllabus  34 

Questions  for  Review 35 

CHAPTER  IV. 

Force  and  Motion 37 

Syllabus 49 

Questions  for  Review 51 

CHAPTER  V. 

The  Mechanical  Powers 52 

Syllabus 62 

Questions  for  Review 64 

1* 


PAGE 

. 9 

14 
. 15 


V 


VI 


CONTENTS. 


CHAPTER  VI.  PAGE 

Geavitatiok 65 

Syllabus 78 

Questions  for  Review 80 

CHAPTER  VII. 

Cohesion  and  Adhesion,  and  Properties  Peculiar  to  Solids  82 

Syllabus 95 

Questions  fob  Review 97 

PART  II. 

FLUIDS. 

CHAPTER  I. 

Hydrostatics 98 

Syllabus 112 

Questions  for  Review Ill 

CHAPTER  II. 

Hydraulics 115 

Syllabus 123 

Questions  foe  Review 125 

CHAPTER  III. 

Pneumatics  . 126 

Syllabus 137 

Questions  foe  Review 139 

PART  III. 

SOUND  AND  HEAT. 

CHAPTER  I. 

The  Cause,  Transmission,  Reflection,  and  Refraction  of 

Sound 140 

Syllabus 152 

Questions  for  Reyhew 153 


CONTENTS. 


vu 


CHAPTES  II.  p^sE 

The  Chaeacteeistics  of  Musical  Souhd. — Musical  Insteumehts  155 

Syllabus 167 

Questions  foe  Review 169 

CHAPTER  III. 

The  Natuee  of  Heat. — Theemohetees  and  Expansion  . 170 

Syllabus 177 

Questions  foe  Review 178 

CHAPTER  IV. 

The  CoiiMUNicATioN  of  Heat. — The  Sueface  Action  of  Bodies  180 

Syllabus 190 

Questions  foe  Review 191 

CHAPTER  V. 

Change  of  State.  — Latent  and  Specific  Heat.  — Mechan- 
ical Equivalent  of  Heat 193 

Syllabus 207 

Questions  foe  Review 208 

PART  IV. 

LIGHT  AND  ELECTRICITY. 

CHAPTER  I. 

Light  : Its  Natuee  and  Soueces. — Action  of  Mattee  on 

Light 210 

Syllabus 227 

Questions  foe  Review 229 

CHAPTER  II. 

Lenses. — Optical  Insteuments  and  Vision  ....  231 

Syllabus 245 

Questions  foe  Review 247 

CHAPTER  III. 

Electeicity. — Electeical  Chaege,  oe  Electeicity  of  High 

Tension 249 

Syllabus 268 

Questions  foe  Review 269 


VI II 


CONTENTS. 


CHAPTER  IV.  PAGE 

CuRREKT  Electricity 271 

Syllabus 279 

Questions  for  Review 280 

CHAPTER  V. 

Properties  of  an  Electrical  Current 281 

Syllabus 286 

Questions  for  Review 287 

CHAPTER  VI. 

Magnetism 288 

Syllabus 295 

Questions  for  Review 296 

CHAPTER  VII. 

Magneto-Electric  Currents.  — Apparatus  Dependent  on 

Electro-Magnets 297 

Syllabus 310 

Questions  for  Review 311 


Natural  Philosophy. 


Part  I. 

Matter  and  Force. 
CHAPTER  I. 

MATTER. 

1.  Matter  is  anything  which  occupies  space,  and 
prevents  other  things  from  occupying  the  same  space. 

If  the  thing  merely  occupies  space,  but  does  not 
prevent  other  things  from  occupying  the  same  space, 
then  it  is  not  matter. 

Both  iron  and  gold  are  kinds  of  matter,  since  they 
occupy  space,  and  prevent  other  pieces  of  matter  from 
being  placed  where  they  are.  The  shadow  of  an  object, 
however,  does  not  prevent  other  things  from  being 
placed  in  it,  and  is  not,  therefore,  a kind  of  matter. 

Both  air  aud  water  are  kinds  of  matter.  When  we  move  other 
bodies  through  them,  these  bodies  do  not  occupy  the  same  place  the 
air  or  the  water  does,  but  merely  push  the  air  or  the  water  out  of  the 
way,  and  then  occupy  the  place  they  have  thus  cleared  for  themselves. 
If  a vessel  be  filled  to  the  brim  with  water,  a stone  dropped  into  it 
will  cause  as  much  water  to  run  out  of  the  vessel  as  will  just  fill  the 
space  the  stone  occupies.  The  same  vessel,  however,  can  be  placed 

9 


10 


NATURAL  PHILOSOPHY. 


where  it  will  be  filled  with  sunlight,  without  any  of  the  water  run- 
ning out. 

2.  The  Senses. — We  acquire  knowledge  bj  means 
of  our  senses,  and  by  them  become  aware  of  the  ex- 
istence of  matter,  which  we  can  see,  feel,  taste,  or 
smell. 

Our  senses  form  the  avenues  or  channels  through  which  impres- 
sions are  received  from  things  outside ; thus,  light  entering  the  eye 
enables  us  to  see  the  peculiarities  of  color  and  form  of  the  object 
from  which  it  came.  By  means  of  the  touch,  we  learn  to  distinguish 
the  nature  of  the  surface  and  the  texture ; by  the  taste  or  smell,  we 
are  enabled  to  select  the  pleasant  and  wholesome  from  the  noxious  ; 
and,  finally,  by  our  hearing,  we  are  enabled  to  understand  the 
thoughts  of  others  when  expressed  in  language. 

3.  Substances.  Elements. — The  different  kind.s  of 
matter  are  called  substances.  Iron,  wood,  water,  milk, 
air,  and  steam  are  substances. 

Substances  are  either  elementary  or  compound.  Ele- 
mentary substances.!  or  elements,  are  those  which  have 
never  been  resolved  into  more  than  one  kind  of 
matter. 

Compound  stibstances  are  those  which  are  formed 
by  the  union  of  tw'o  or  more  elemexttary  substances. 

Gold  is  an  elementary  substance.  We  cannot,  by 
any  known  means,  break  it  up  or  separate  it  into 
anjffhing  but  gold.  Brass  is  a compound  substance, 
since  we  can  separate  it  into  copper  and  zinc ; or,  by 
melting  copper  and  zinc  together  in  the  right  propor- 
tions, Ave  can  produce  brass. 

All  the  compound  substances  in  the  Avorld  are 
formed  by  various  combinations  of  about  seA'entA' 
elementary  substances. 

Any  definite  piece  of  matter  is  called  a body. 
Bodies  may  be  either  large  or  small ; thus,  both  the 


MA  TTER. 


11 


eartli  and  a grain  of  sand  are  bodies,  since  they  both 
are  pieces  of  matter. 

4.  Changes  of  Matter.  — All  kinds  of  matter  have 
different  peculiarities  or  properties,  by  means  of  which 
we  are  enabled  to  recognize  them.  Thus,  gold  can  be 
readily  distinguished  from  marble,  since  its  color  and 
weight  are  different ; and  then,  too,  it  can  be  drawn 
out  into  wire,  or  beaten  into  thin  sheets,  while  marble 
cannot.  They  differ  also  in  other  respects. 

Matter  is  constantly  undergoing  change.  This 
change  is  of  two  distinct  kinds,  viz. : 

1st.  Physical  change,  or  that  which  may  occur 
without  the  loss  of  those  peculiarities  or  proper- 
ties by  which  we  recognize  the  substance ; and, 

2d.  Chemical  change,  or  that  which  cannot  occur 
without  the  loss  of  such  peculiarities  or  properties. 

As  an  example  of  a physical  change,  we  may  take  a piece  of  steel, 
such  as  a common  pen,  and  after  examining  it  carefully,  so  as  to  note 
its  peculiarities,  ruh  it  once  or  twice  against  a magnet.  The  pen  will 
now  have  acquired  a property  it  did  not  possess  before ; it  will  attract 
iron  filings  to  it ; but  if  we  again  examine  the  pen  carefully,  we  can- 
not see  that  it  has  lost  any  of  the  properties  it  previously  possessed. 

If,  however,  we  expose  the  pen  for  some  time  to  damp  air,  it  will 
become  covered  with  a thick,  brown  rust,  which  is  formed  by  oxygen, 
a substance  in  the  air,  combining  with  the  iron  of  the  pen.  Now  the 
rust  so  formed  in  no  way  resembles  either  of  the  things  out  of  which 
it  was  formed,  since  one  of  them  was  the  pen  itself,  and  the  other  an 
invisible  gas.  This  change,  then,  is  a chemical  change,  since  both 
bodies  have  lost  the  properties  or  peculiarities  by  which  they  are  gen- 
erally recognized. 

5.  Physics  or  Natural  Philosophy,  and  Chemistry. 

— Natural  Philosophy  or  Physics  is  that  study  tvbicb 
considers  the  causes  and  effects  of  the  physical  changes 
to  which  matter  is  subject. 

Chemistry  is  that  study  which  considers  the  causes 


12 


NATURAL  PHILOSOPHY. 


and  effects  of  tlie  chemical  changes  to  which  matter 
is  subject. 

6.  Phenomena. — Anything  which  happens  in  the 
ordinary  course  of  nature  is  called  a phenomenon.  As 
the  word  is  commonly  used,  it  means  something 
unusual  or  strange ; as  used  in  science,  it  means  any- 
thing which,  happens  naturally.  The  fall  of  a leaf, 
the  shining  of  the  sun,  the  fall  of  a rain-drop,  or  the 
growth  of  a plant,  are  all  natural  phenomena. 

7.  Cause  and  Effect.  — Nothing  happens  of  itself. 
All  natural  phenomena  are  produced  by  certain 
causes ; for  example,  unsupported  bodies  fall  to  the 
earth : here  all  we  see  is  the  effect,  namely,  the  motion. 
The  cause  of  the  motion  is  the  attraction  which  the 
earth  has  for  the  body. 

An  effect  may  itself  be  the  cause  of  some  succeed- 
ing effect ; thus,  the  body,  in  falling  to  the  earth,  may 
give  some  of  its  motion  to  another  body  which  it 
strikes,  and  this  effect  may  in  turn  be  the  cause  of 
some  other  effect,  and  so  on  indefinitely. 

The  causes  which  produce  natural  phenomena  can 
all  be  traced  to  certain  forces,  one  of  the  most  import- 
ant of  which  is  heat. 

8.  Natural  Law.  — If  we  observe  any  natural  phe- 
nomenon, we  will  notice  that  the  relation  between  cause 
and  effect  is  constant  and  invariable.  The  same  cause, 
acting  in  the  same  way,  alwavs  produces  the  same 
effect.  Thus,  unsupported  bodies  always  fall  to  the 
earth ; a steel  pen  rubbed  against  a magnet  alwa3"s 
becomes  magnetic,  and  acquires  the  property  of  at- 
tracting iroiT  filings ; the  same  pen,  unless  specialh^ 
protected,  alwaj's  becomes  covered  with  rust  when 
exposed  for  some  time  to  damp  air. 


MA  TTER. 


13 


When,  by  observation,  we  have  discovered  the  cause 
of  any  natural  phenomenon,  and  have  ascertained  the 
order  in  which  cause  and  effect  follow  each  other,  this 
order,  expressed  in  language,  forms  what  is  called  a 
natural  law. 

Natural  Philosophy  has  for  its  object  the  study  of  natural  laws. 
This  definition,  it  will  be  seen,  embraces  the  study  of  all  natural  phe- 
nomena; but  natural  philosophy  is  generally  restricted  to  the  study 
of  the  laws  concerning  the  physical  changes  that  take  place. 

9.  Method  of  Study.  Experiment, — There  is  only 
one  way  by  which  we  can  discover  natural  laws,  and 
that  is  by  observation.  If  we  wish  to  know  what 
effect  will  follow  a certain  cause,  we  must  make  the 
trial,  and  observe  what  happens.  If,  after  repeated 
trials,  we  obtain  the  same  effects,  we  may  conclude  that 
Ave  have  discovered  the  law. 

10.  Force  and  Energy.  — All  changes  in  nature 
are  caused  by  the  action  of  certain  forces.  The  term 
energy  is  employed  to  designate  the  amount  of  work 
Avhich  can  be  accomplished  by  the  action  of  any  force. 

By  the  muscular  force  of  the  arm,  a pound  weight 
may  be  raised  a certain  distance,  say  one  foot;'  if  the 
same  force  continue  to  act,  the  Aveight  may  be  raised 
two  feet.  Here,  in  each  case,  the  cause  of  the  motion 
is  the  same,  viz.,  the  muscular  force  of  the  arm ; but 
the  energy,  or  the  amount  of  Avork  done,  is  tAvice  as 
great  in  the  latter  case  as  in  the  former. 

11.  Indestructibility  of  Matter  and  Energy. — It 

is  believed  that  there  exists  in  the  universe  a certain 
definite  quantity  of  matter  and  of  energj'’.  Neither 
of  these  can  by  any  known  means  be  increased  or 
diminished.  Matter  and  energy  may,  during  changes, 
disappear,  but  only  to  reappear  in  other  forms.  Thus, 
2 


14 


NATURAL  PHILOSOPHY. 


when  a piece  of  paper  or  wood  is  burned,  it  disap- 
pears, and  the  heat  set  free  is  apparently  lost  ; but 
the  paper  or  wood  has,  by  the  process  of  burning, 
been  changed  into  invisible  gases,  and  the  heat  set 
free  has  acted  on  the  matter  around  it,  and  produced 
changes  therein.  Matter  may  change  from  one  form 
to  another,  and  energy  may  be  converted  into  some 
other  form  of  energy,  but  neither  can  be  destroyed. 

Syllabus. 

Matter  is  anything  which  occupies  space  and  prevents  other  things 
from  occupying  the  same  space. 

No  two  pieces  of  matter  can  be  in  the  same  place  at  the  same  time, 
for  all  matter  fills  the  space  it  occupies,  and  prevents  other  matter  from 
occupying  the  same  space. 

Air  and  water  are  kinds  of  matter,  since  they  occupy  space  and 
exclude  other  bodies  from  the  space  they  occupy.  We  know  of  the 
existence  of  matter  by  means  of  our  senses ; we  can  see,  feel,  smell,  or 
taste  it. 

Different  kinds  of  matter  are  called  substances;  substances  are 
either  elementary  or  compound.  Elementary  substances  are  called 
elements. 

Matter  is  constantly  undergoing  change.  A physical  change  is  one 
which  may  happen  without  the  substance  losing  those  peculiarities  or 
properties  which  generally  enable  us  to  recognize  it.  A chemical 
change  is  one  which  obliterates  or  removes  such  properties. 

Physics  or  Natural  Philosophy  studies  the  causes  and  efiects  of 
physical  changes ; chemistry  studies  the  causes  and  effects  of  chemical 
changes. 

A phenomenon  is  a natural  event,  or  anything  which  occurs  in  the 
usual  course  of  nature. 

Nothing  happens  without  a cause.  The  causes  which  produce  natural 
phenomena  may  be  traced  to  certain  forces,  one  of  the  most  important 
of  which  is  heat. 

A natural  law  is  that  which  expresses  the  order  in  which  cause  and 
effect  follow  each  other.  Natural  laws  are  discovered  by  means  of 
observation. 


QUESTIONS  FOR  REVIEW. 


15 


By  the  term  energy,  we  mean  the  amount  of  work  which  can  he 
accomplished  by  the  action  of  any  force,  flatter  and  energy  are 
both  indestructible.  During  changes,  matter  and  energy  may  both 
disappear,  but  only  to  reappear  in  some  other  forms. 


Questions  for  Review. 

How  can  you  tell  whether  the  things  you  see  are  composed  of  matter 
or  not  ? Prove  that  both  air  and  water  are  forms  of  matter.  Is  a 
shadow  a form  of  matter  ? Why  ? 

How  do  we  become  acquainted  with  different  kinds  of  matter? 
What  are  substances?  Name  some  different  substances.  What  are 
elementary  substances  ? What  are  compound  substances  ? Give  some 
examples  of  each.  What  is  a body  ? 

What  do  you  understand  by  the  properties  of  a body  ? To  what 
two  different  kinds  of  change  is  matter  subject?  What  is  the  differ- 
ence between  these  changes  ? Give  an  example  of  each  of  these 
kinds  of  change.  What  is  Physics  or  Natural  Philosophy  ? What  is 
chemistry  ? 

What  is  the  difference  between  the  scientific  and  the  common  use  of 
the  word  pheuomeuon?  Name  some  different  natural  phenomena. 
What  do  you  understand  by  the  cause  of  a natural  phenomenon  ? 
What  is  an  effect  ? What  relation  always  exists  between  cause  and 
effect?  Name  any  important  cause  which  produces  many  natural 
phenomena. 

Define  natural  law.  Give  an  example  of  any  natural  law.  What 
has  the  study  of  natural  philosophy  to  do  with  natural  law  ? In 
what  way  only  can  any  law  of  nature  be  discovered  ? 

Define  energy.  Can  the  amount  of  either  matter  or  energy  which 
now  exists  in  the  universe  be  increased  or  diminished?  What  is  in- 
destructibility ? 


CHAPTER  II. 

GENERAL,  PROPERTIES  OF  MATTER. 

12.  Names  of  General  Properties. — Bj  the  prop- 
erties of  a body  we  understand  those  peculiarities  or 
qualities  which,  enable  ns  to  recognize  it.  Properties 
are  either  general  or  specific.  General  properties  are 
those  which  are  common  to  all  matter.  Specific  or 
particular  properties  are  those  which  are  possessed 
only  by  certain  kinds  of  matter. 

The  most  important  of  the  general  properties  of 
matter  are  magnitude  or  extension,  impenetr ability, 
divisibility,  porosity,  compressibility , expjansibility,  mo- 
bility, and  inertia. 

13.  Magnitude  or  Extension. — All  matter  occupies 
space  or  fills  room,  that  is,  it  has  size.  This  property 
is  generally  knoAvn  by  the  name  of  magnitude  or  ex- 
tension. Matter  extends  or  fills  space  in  three  difterent 
directions,  namely,  in  length,  in  breadth,  in  thickness  ; 
or,  in  other  words,  all  matter  possesses  volume. 

14.  Units  of  Measurements. — In  this  country,  the 
dimensions  of  a,  body  are  measured  in  inches,  feet, 
yards,  or  miles.  In  France  and  Europe,  generallj', 
dimensions  are  measured  in  metres  or  in  decimals  or 
multiples  of  a metre. 

The  following  tables  give  the  values  of  these  units, 
viz. ; 


16 


GENERAL  PROPERTIES  OF  MATTER.  17 


English  Measure. 

Measures  of  Length.  Measures  of  Surface.  Measures  of  Volume. 

12  in.  make  one  ft.  144  sq.  in.  =1  sq.  ft.  1728  cub.  in.=  1 cub.  ft. 

3 ft.  “ “ yd.  9 sq.  ft.  = 1 sq.  yd.  27  cub.  ft.  = 1 cub.  yd. 

1760  yds.,  or  5280  ft., 


make  one  mile. 

French  Measure. 

Measures  of  Length. 


1 metre equals 

39.37 

Eng.  in.,  or  3.280  ft. 

1 decametre,  or  10  metres  “ 

393.7 

1 hectometre,  or  100  metres  “ 

3937. 

((  C( 

1 kilometre,  or  1000  “ “ 

39370. 

“ “ or  1093.6  yds. 

1 decimetre,  or  y’j-  “ “ 

3.937 

1 centimetre,  or  “ “ 

.3937 

1 millimetre,  or  '*  " 

.03937 

“ “ or  in.  nearly. 

Measures  of  Area  or  Surface. 

1 square  metre  equals  . . . 1550.06  sq.  in.,  or  10.764  sq.  ft. 

1 square  decimetre  equals  . 15.5006  “ “ 

1 square  centimetre  “ . .155006  “ “ 

1 square  millimetre  “ . .00155006  “ “ 

Measures  of  Volume. 

1 cubic  metre  equals  . . . 61027.1  cub.  in., or  35.3166  cub.ft. 

Icubicdecimetre,  or  litre,  equals  61.0271  “ “ .2202  gals. 

1 cubic  centimetre  equals  .0610271  “ “ 


The  measures  of  surface  are  obtained  by  squaring  the  measures  of 
length.  Thus,  12  ins.  X 12  = 144  square  inches,  or  one  square  foot ; 
3 ft.  X3  = 9 square  feet,  or  one  square  yard;  39.37  ins.  X 39.37  = 
1550.06  square  inches,  or  one  square  metre. 

The  measures  of  volume  are  obtained  by  cubing  the  measures  of 
length;  thus,  12  ins.  X 12  X 12  = 1728  cubic  inches,  or  one  cubic  foot. 

The  following  table  will  be  found  convenient  in  changing  English 
into  French  units : 


1 inch 
1 foot 
1 yard 
1 mile 


25.40  millimetres. 
.3048  metre. 

.9144  metre. 

1.609  kilometres. 


The  value  of  one  square  inch  can  be  obtained  by  squaring  25.40 
millimetres  = 645.16  sq.  millimetres.  The  value  of  one  cubic  inch 
can  be  obtained  by  cubing  25.40  =16387.06  cubic  millimetres. 

2*  B 


18 


NATURAL  PEILOSOPHY. 


Figure  1 represents  the  actual  lengths  of  the  French  decimetre,  and 
Figure  2 of  four  English  inches.  Since  a decimetre  equals  3.937 
^ inches,  it  is  but  a trifle  shorter  than  four  English 
inches.  From  to  i?  is  one  decimetre;  from  A 
to  a is  one  centimetre,  of  which  there  are  ten  in 
one  decimetre,  or  in  the  whole  length  of  A B. 
From  .A  to  6 is  one  millimetre,  of  which  there 
are  ten  in  every  centimetre. 

From  C to  D is  four  English  inches;  from  C 
S _ to  a is  one  inch ; from  C to  i is  one-tenth  of  an 

inch.  There  are  about  twenty-five  millimetres 
in  one  inch. 


15.  Impenetrability  is  that  prop- 
erty wliicli  prevents  any  ttvo  particles 
of  matter  from  occupying  the  same 
space  at  the  same  time. 

As  we  have  already  seen,  magni- 
tude or  extension  and  impenetrability 
are  necessary  to  the  existence  of  mat- 
ter. They  are^  therefore,  sometimes 
called  the  essential  properties  of  matter. 


Experiment.  — If  an  empty  glass  goblet  be 
held,  mouth  downwards,  in  a vessel  of  water,  it 
will  be  found  that  the  water  will  not  rise  and  fill 
the  goblet,  since  it  is  already  full  of  air.  This 
proves  that  air  and  water  cannot  be  in  the  same 
space  at  the  same  time.  If,  however,  a vessel 
open  at  both  ends,  as,  for  example,  a glass  lamp 
^ chimney,  be  dipped  into  water,  it  will  be  found 

that  the  water  will  rise  as  high  inside  the  chimney  as  it  is  on  the 
outside,  for,  as  the  water  rises  in  the  chimney,  it  pushes  the  air  out 
at  the  top. 

Caution. — Unless  the  goblet  be  held  with  its  mouth  quite  level  with 
the  water,  some  of  the  air  will  escape,  and  mar  the  experiment. 

Although,  in  many  cases,  two  different  bodies  seem  to  occupy  the 
same  place,  yet,  in  reality,  this  is  not  the  case.  Like  the  body  moving 
through  air  or  water,  they  merely  move  parts  of  the  other  body  out 
of  the  space  they  occupy.  Thus,  a nail  driven  into  a board  can  only 
enter  it  by  crowding  some  of  the  particles  of  the  board  more  clc.«ely 


GENERAL  PROPERTIES  OF  31  ALTER.  19 


together.  If,  too,  in  the  experiment  just  described,  the  goblet  be 
pushed  far  down  into  the  water,  the  water  will  rise  some  little  dis- 
tance inside  it,  because  the  pressure  has  packed  the  particles  of  air 
more  closely  together. 

16.  Divisibility  is  tliat  property  of  matter  wlrich 
enables  us  to  cut  it  up  or  divide  it  into  a number  of 
smaller  pieces.  So  far  as  simple  experiment  shows, 
there  would  appear  to  be  no  practical  limit  to  the  di- 
visibility of  matter.  AYe  can  continue  to  divide  it 
until  the  particles  are  too  small  to  be  directly  seen,  and 
even  then,  by  using  a microscope,  we  can  carry  the  di- 
vision still  further. 

The  following  examples  will  show  the  wonderful  extent  to  which  it 
is  possible  to  carry  the  division  of  matter.  Gold  can  be  beaten  into 
leaves  so  thin  that  it  would  take  about  three  hundred  thousand  of 
such  leaves,  placed  one  upon  the  other,  to  make  a pile  one  inch  thick, 
and  yet  each  of  these  leaves  can  be  cut  into  very  small  pieces. 

A grain  of  musk  will  continue  for  yeans  to  give  off  its  odorous 
particles  to  the  air  around  it,  without  decreasing  very  perceptibly 
in  weight. 

A very  small  quantity  of  certain  coloring  matters  will  give  a de- 
cided tint  to  a large  quantity  of  water.  Now,  since  the  water  so 
colored  may  be  divided  into  a very  great  number  of  parts,  in  each  of 
which  the  color  is  distinctly  visible,  the  quantity  of  coloring  matter 
in  each  must  be  exceedingly  minute. 

The  microscope  has  revealed  to  us  the  existence  of  animalcules  so 
small  that  millions  can  easily  swim  about  in  the  space  of  a cubic  inch 
without  touching  one  another. 

17.  Atoms  and  Molecules. — Notwithstanding  the 
above,  and  other  wonderful  instances  of  the  extreme 
divisibility  of  matter,  we  have  reasons  for  believing 
that  matter  is  not  infinitely  or  indefinitely  divisible, 
but  that  if  we  could  continue  to  divide  a piece  of  mat- 
ter far  enough,  we  would  reach  a limit  beyond  which  it 
would  be  impossible  further  to  divide  it.  These  final 
particles  of  matter  are  called  atoms.  The  word  atom 
means  that  which  cannot  be  cut. 


20 


NATURAL  PHILOSOPHY. 


Since  we  know  hj  actual  trial  that  we  can  divnde 
matter  into  very  small  particles,  the  atoms  must  be 
exceedingly  small;  but  of  their  real  nature,  no  definite 
knowledge  has  as  yet  been  obtained.  The  atoms  sel- 
dom, if  ever,  exist  separately,  but  combine  with  one 
another,  and  form  groups  of  two  or  more  atoms,  called 
molecules.  The  force  which  causes  the  atoms  to  com- 
bine is  the  chemical  force.  Molecules,  though  larger 
than  atoms,  are  still  exceedingly  small  — smaller,  in- 
deed, than  the  smallest  particles  into  which  we  have 
been  able  to  divide  bodies. 

The  molecule  of  a compound  substance  is  the  small- 
est possible  quantity  of  that  substance  that  can  exist. 
Water  is  a compound  substance,  composed  of  two 
atoms  of  hyi'lrogeu  combined  with  one  atom  of 
oxygen.  Any  compound  containing  less  than  two 
atoms  of  hydrogen  combined  with  one  atom  of  ox}'- 
gen,  would  not  be  water.  The  molecule  of  water,  or 
the  smallest  quantity  of  water  that  can  exist,  will 
therefore  contain  two  atoms  of  h_\xlrogen  and  one  of 
oxygen.  The  molecule  of  an  elementary  body  consists 
of  a group  of  atoms  of  that  body. 

The  very  small  particles  of  matter  obtnined  artificially,  by  con- 
tinued division  or  by  grinding,  are  neither  atoms  nor  molecules. 
The  word  particle  is  sometimes  used  in  the  sense  of  atom  or  molecule ; 
such  use  is  only  correct  when  we  mean  the  smallest  possible  particle. 

18.  Porosity. — Neither  the  atoms  nor  the  molecules 
touch  one  another;  though  they  are  nearer  together 
in  solids  than  in  liquids  or  gases,  yet  it  is  believed 
they  are  not  in  actual  contact,  even  in  the  most  solid 
substances  known.  The  spaces  tvliich  separate  them 
are  called  pores.  The  size  of  the  pores  varies  greatly. 
In  some  kinds  of  matter,  the  pores  can  readily  be 
seen,  as,  for  example,  in  most  woods,  or  in  sponges. 


GENERAL  PROPERTIES  OF  MATTER.  21 


In  otliers  we  cannot  see  tlie  pores,  even  Avitli  tlie  aid 
of  a microscope.  A¥e  can,  however,  in  many  cases, 
show  that  they  exist,  by  forcing  liquids  through  them. 
Thus,  if  water  be  placed  in  a strong  vessel  of  gold, 
and  a powerful  pressure  be  exerted  on  the  water,  it 
can  be  forced  to  pass  through  the  vessel  without  rup- 
turing it.  The  water  must  therefore  have  passed 
through  the  pores. 

19.  Compressibility.  — All  matter,  when  subjected 
to  sufficient  pressure,  can  be  made  to  occupy  a smaller 
space ; or,  in  other  words,  all  matter  possesses  the 
property  of  compressihility . Substances  vary  in  their 
degree  of  compressibility.  Of  the  three  conditions  in 
which  matter  is  found,  gases  are  the  most  compressi- 
ble, and  solids  the  least.  The  compressibility  of  most 
solids  and  liquids  is  very  small,  considerable  force 
being  required  to  produce  a perceptible  change  in 
their  volume.  The  compressibility  of-  gases  is  very 
much  greater  than  that  of  solids  or  liquids. 

All  matter,  when  cooled,  decreases  in  bulk.  This 
decrease  is  called  contraction.,  and  is  to  be  distinguished 
from  compression. 

As  a rule,  gases  contract  more  than  liquids,  and 
liquids  more  than  solids,  on  the  loss  of  equal  quan- 
tities of  heat. 

20.  Expansibility.  — • As  matter  contracts  or  de- 
creases in  bulk  by  a loss  of  heat,  so  also  it  expands  or 
increases  in  bulk  by  an  increase  of  heat.  Gases  ex- 
pand more  than  liquids,  and  liquids  more  than  solids, 
by  the  same  change  of  temperature. 

To  the  above  general  statement  there  exist  exceptions,  a few 
substances  expanding  on  a loss,  and  cortracting  on  an  increase,  of 
heat. 


22 


NATURAL  PHILOSOPHY. 


If,  as  is  believed,  the  size  of  neither  the  atoms  nor  the  molecules  is 
changed  during  expansion  or  contraction,  then  the  increase  or  decrease 
in  the  bulk  so  produced  can  only  be  due  to  the  increase  in  the  size  of 
the  pores  or  spaces  between  the  atoms  and  molecules.  As,  therefore, 
all  matter  expands  or  contracts  when  subjected  to  changes  of  tem- 
perature, all  matter  must  contain  pores. 

Experiment. — If  an  empty  glass  bottle  be  held,  mouth  downwards, 
in  a plate  of  w'ater,  so  that  the  mouth  is 
just  under  the  water,  and  the  hand  be  made 
to  cover  as  much  of  the  outside  of  the  bottle 
as  possible,  the  heat  of  the  hand  will  cause 
the  air  inside  the  bottle  to  expand,  so  that 
the  bottle  will  no  longer  be  able  to  hold  it 
all,  and  it  will  be  seen  to  bubble  out  from 
the  mouth  of  the  bottle. 

Caution. — The  bottle  should  be  compar- 
atively large,  so  as  to  hold  a fair  amount  of 
air,  and  made  of  glass  as  thin  as  possible. 

Pig,  3.-Expansion  of  Air. 

quickly  communicated  to  the  air  within. 
If  the  bottle  be  dipped  too  far  down  in  the  water,  it  will  take  a longer 
time  for  the  bubbles  of  air  to  escape,  on  account  of  the  pressure  of 
the  water  on  the  outside. 

21.  Mobility  is  tliat  property  of  matter  -vs-iiicli  en- 
ables it  to  be  moved  or  to  change  its  place. 

Since  the  earth  is  ahvays  rotating  on  its  axis  and 
revolving  around  the  sun,  it  is  clear  that  nothing  on 
the  earth  is  ever  actually  at  rest.  We  generally  say, 
however,  that  a body  is  at  rest  when  it  is  not  chang- 
ing its  position  as  regards  neighboring  bodies. 

Besides  the  more  apparent  motions  that  occur  around  us.  like  the 
flowing  of  a river,  the  flight  of  a bird,  or  the  fall  of  a stone,  there  are 
other  motions  too  minute  to  be  seen.  The  molecules  are  never  at  rest, 
but  are  in  rapid  motions  towards  and  from  each  other.  These  motions 
cause  various  phenomena  of  heat  and  light. 

22.  Inertia. — A body  never  begins  to  move,  stops 
moving,  or  changes  the  direction  in  Avhich  it  has  been 
moving,  unless  force  of  some  kind  acts  upon  it.  In 


GENERAL  PROPERTIES  OF  MATTER. 


23 


other  words,  matter  can  do  nothing  of  itself  towards 
changing  its  condition,  either  of  rest  or  motion. 

If  we  attempt  to  move  a comparatively  large  body 
from  a state  of  rest,  as,  for  example,  a wheel  revolving 
freely  on  an  axis,  we  will  find  it  necessary  to  exert 
onr  strength  for  some  time  before  we  can  get  the 
wheel  to  move  rapidly ; that  is,  we  find  that  a body  at 
rest  apparently  offers  a resistance  to  changing  its  state 
of  rest.  On  the  other  hand,  when  the  wheel  has  been 
set  in  motion,  we  will  also  find  it  necessary  to  exert 
our  strength,  but  this  time  in  the  opposite  direction, 
before  we  can  bring  the  body  to  rest  again ; that  is, 
we  find  that  a body  in  motion  apparently  offers  a re- 
sistance to  chano'ino-  its  state  or  condition  of  motion. 

O O 

By  the  inertia  of  matter  we  understand  its  tendency 
to  continue  in  whatever  condition  it  may  be,  whether 
of  rest  or  of  motion,  until  some  force  acts  upon  it. 
Therefore  it  follows,  from  the  property  of  inertia, 

1st.  That  a body  at  rest  tvill  continue  at  rest  forever., 
unless  some  force  acts  upon  it ; and, 

2d.  That  a body  in  motion  will  continue  moving  in 
a straight  line  forever,  unless  some  force  acts  upon  it. 

It  is  very  easy  to  believe  that  a body  once  at  rest 
will  continue  at  rest  forever,  until  acted  on  by  some 
force,  since  such  is  our  common  experience.  But  it  is 
difficult,  at  first,  to  believe  that  the  same  thing  is  true 
of  a body  in  motion.  If,  for  example,  we  throw  a 
stone  straight  up  in  the  air,  we  know  that  it  will 
not  continue  moving  upwards  forever.  In  reality,  it 
moves  more  and  more  slowly  every  moment,  and  at 
last  entirely  stops,  and  begins  to  fall  to  the  earth. 
But  in  this  case  the  body  does  not  stop  its  own  mo- 
tion. It  is  stopped  because  the  earth  is  constantly 
pulling  it  down  towards  it,  and  because  the  air  is 


24 


NATURAL  PHILOSOPHY. 


resisting  its  motion.  Could  we  go  out  into  tlie  empty 
space,  beyond  the  influence  of  any  other  force,  and 
throw  the  stone  in  any  direction,  it  w’ould  continue 
moving  in  a straight  line  in  that  direction  forever, 
since,  as  it  is  inert,  it  has  no  more  force  or  power  to 
stop  its  motion  than  it  has  to  begin  to  move. 

fl’he  earth  moves  through  the  apparently  empty 
space  around  the  sun,  and,  since  there  is  nothing  to 
stop  its  motion,  must  continue  moving  forever. 

23.  Examples  of  Inertia. — When  a train  of  cars 
begins  to  move,  it  takes  some  time  before  it  reaches 
its  full  speed,  on  account  of  the  inertia  of  the  train. 
When,  hoAvever,  the  train  has  attained  this  speed,  con- 
siderable force  must  be  exerted  to  stop  it. 

Before  a moving  body  can  be  brought  to  rest,  it 
must  lose  an  amount  of  energy  equal  to  that  wdiich 
caused  its  motion.  Cannon-balls  owe  their  great  de- 
structive power  to  the  fact  that  they  have  been  set 
in  motion  by  the  explosion  of  gunpowder,  and  hence 
will  overcome  considerable  resistance  before  stopping. 

If  w'e  jump  from  a rapidly  moving  coach,  w'e  are 
very  apt  to  fall  over,  because,  on  reaching  the  ground, 
the  motion  of  our  feet  is  stopped  while  the  rest  of 
our  body  continues  to  move  forward.  A running  jump 
will  carry  us  much  farther  than  a standing  jump,  be- 
cause, if  we  first  run  rapidly  in  the  direction  in  which 
we  wish  to  jump,  the  motion  thus  given  to  the  body 
wdll  help  to  carry  it  in  that  direction. 

Experiment. — If  a book  be  stood  upright  on  its  end  on  a sheet  of 
paper,  it  will  be  found  that,  if  the  paper  be  slowly  pulled,  the  book 
can  be  moved  without  falling,  since,  in  this  case,  the  motion  is 
gradually  imparted  to  it.  If,  now,  while  the  book  is  moving,  we 
suddenly  stop  pulling  the  paper,  the  book  will  fall  forwards,  since  its 
top  keeps  on  moving  after  the  part  resting  on  the  paper  has  stopped. 


GENERAL  PROPERTIES  OF  MATTER.  25 


If,  on  starting,  we  pnll  the  paper  quickly,  the  hook  will  fall  back- 
wards, because  the  part  resting  on  the  paper  is  moved  forwards  be- 
fore the  top  has  commenced  to  move. 

Caution.  — The  experiment  will  be  more  impressive  and  apt  to  suc- 
ceed, if  a long,  heavy  book  be  used. 

24:.  Living  Bodies  Possess  Inertia. — Living  bodies, 
as  well  as  those  without  life,  possess  inertia. 

We  are  conscious  of  having  to  exert  oitr  strength 
before  we  can  move  about  from  place  to  place.  Thus, 
if  we  move  an  arm,  we  do  it  by  means  of  the  muscular 
strength  or  force  of  the  arm.  When  we  have  once 
set  our  bodies  in  rapid  motion,  as,  for  example,  in 
running  down  hill,  we  find  it  necessary  to  exert  con- 
siderable strength  in  order  to  stop  suddenly. 

25.  Resistances  to  Motion.  — A body  moving- 
through  air  or  water  can  only  advance  by  pushing 
the  air  or  water  out  of  its  way.  Since  both  air 
and  water  possess  inertia,  they  cannot  move  them- 
selves out  of  the  way,  and  therefore  require  that 
force  act  upon  them.  This  force  is  taken  from  the 
moving  body,  and  diminishes  its  motion.  Eesistances 
of  this  kind  are  called  fluid  resistances.,  and  are  im- 
pediments to  motion. 

When  bodies  are  slid  or  rolled  over  one  another, 
they  meet  another  resistance  or  impediment  to  motion. 
Even  the  smoothest  surfaces  we  can  obtain  are  full  of 
irregularities ; and  when  one  body  is  slid  or  rolled  over 
another,  the  projections  of  the  one  fitting  into  the  de- 
pressions in  the  otlier,  cause  a resistance  to  moticn. 
Besides  the  irregularities  of  the  surfaces,  whenever 
two  bodies  are  brought  near  each  other,  they  attract 
or  tend  to  hold  on  to  each  other.  The  re.sistances  to 
motion  produced  in  this  Avay  are  called  /Vfc/ioTj,. 


26 


NATURAL  PHTLOSOPIIY. 


Friction,  therefore,  is  caused 

1st.  By  tlie  irregularities  of  the  surfaces  in  contact; 
and, 

2d.  By  the  attraction  which  the  bodies  have  for  each 
other. 

.ao>»»4o<> 


Syllabus. 

By  the  properties  of  a body,  we  mean  those  peculiarities  or  qual- 
ities which  enable  us  to  recognize  it.  Properties  are  either  general  or 
specific. 

Magnitude  or  extension,  impenetrability,  divisibility,  porosity,  com- 
pressibility, expansibility,  mobility,  and  inertia  are  the  most  import- 
ant of  the  general  properties  of  matter. 

Magnitude  and  impenetrability  are  sometimes  called  the  essential 
properties  of  matter,  because  they  are  necessary  to  its  existence. 

The  unit  of  measurement  used  in  this  coantrj’’  is  the  inch  and  its 
multiples;  that  commonly  used  in  France,  and  thronghout  Europe, 
is  the  metre  and  its  multiples,  or  subdivisions.  One  decimetre  is 
very  nearly  equal  to  four  English  inches. 

The  atoms  and  molecules  are  impenetrable.  When  a nail  is  driven 
into  wood,  or  a stone  is  dropped  into  water,  the  molecules  are  pushed 
aside,  not  penetrated. 

Although,  so  far  as  experiment  is  concerned,  our  abilit}'  to  divide 
matter  appears  to  be  without  limit,  yet  we  believe  that,  if  the  division 
were  carried  far  enough,  tve  would  at  length  reach  minute  particles 
that  could  no  longer  be  divided.  These  particles  are  called  atoms. 

The  atoms  are  believed  to  be  unalterable,  and  their  size  unaffected 
by  heat  or  cold.  The  atoms  seldom,  if  ever,  exist  separately,  but  com- 
bine with  one  another,  and  form  groups  of  atoms  called  molecules. 
Neither  the  atoms  nor  the  molecules  touch  one  another,  even  in  the 
densest  kinds  of  matter.  The  spaces  between  the  atoms  or  the  mole- 
cules are  called  pores. 

We  know  that  even  solid  matter  contains  pores,  1st,  Because  many 
solids  are  permeable  to  liquids ; 2d.  Because  all  matter  is  compressi- 
ble ; 3d.  Because  all  matter  contracts  by  cold. 

When  matter  expands  or  contracts  by  heat  or  cold,  or  when  it  is 
compressed,  it  is  not  the  size  of  the  atoms  or  molecules  which  is 
changed,  but  the  size  of  the  spaces  or  pores  between  the  atoms  or 
molecules. 


QUESTIONS  FOR  REVIEW. 


27 


The  expansion  or  contraction  of  gases  is  greater  than  that  of  liquids, 
and  that  of  liquids  greater  than  that  of  solids. 

Since  the  earth  is  constantly  moving,  nothing  on  its  surface  is  at 
actual  rest.  We  generally  consider,  however,  that  a body  is  at  rest 
when  it  is  not  changing  its  position  as  regards  neighboring  bodies. 

A body  never  begins  to  move,  stops  moving,  or  changes  the  direc- 
tion of  its  motion,  unless  force  acts  upon  it. 

By  inertia  we  understand  the  tendency  matter  has  of  continuing 
in  whatever  state  it  may  be,  whether  of  rest  or  motion,  until  some 
force  acts  upon  it ; therefore,  a body  at  rest  will  continue  at  rest  for- 
ever, or  a body  in  motion  will  continue  in  motion  forever,  unless 
force  acts  upon  it. 

Living  bodies  possess  inertia.  Force  is  necessary  to  move  our 
bodies  or  to  stop  their  motion.  A body  moving  through  air  or  water 
loses  all  the  energy  it  gives  to  the  air  or  the  water  it  pushes  out  of  its 
way. 

Friction  is  the  resistance  which  one  body  experiences  in  being  slid 
or  rolled  over  another,  and  is  caused  by  the  irregularities  of  the  sur- 
faces, and  the  attractions  the  two  bodies  have  for  each  other. 


Questions  for  Review. 

What  is  meant  by  the  properties  of  a body  ? What  is  the  difference 
between  general  and  specific  properties  ? Which  two  general  proper- 
ties are  sometimes  called  the  essential  properties  ? Name  the  general 
properties  of  matter. 

Define  magnitude  or  extension.  What  unit  of  measurement  is  gen- 
erally used  in  this  country  ? In  Europe  ? 

Define  impenetrability.  Is  it  the  atoms  and  molecules  or  the  pores 
that  are  impenetrable?  Is  matter  divisible  without  limit?  Can  we 
prove  this  by  experiment?  Name  any  instances  of  the  extreme  di- 
visibility of  matter. 

Define  atom  and  molecule.  What  are  pores?  How  do  we  know 
that  pores  exist  ? 

Which  is  the  more  compressible,  solids  or  liquids  ? Liquids  or  gases  ? 
When  matter  is  compressed,  is  it  the  atoms  and  the  molecules  or  the 
pores  which  are  made  smaller  ? Which  is  the  more  expansible,  solids 
or  liquids  ? Liquids  or  gases  ? 

Describe  in  full  any  experiment  by  means  of  which  the  expansibility 
of  air  may  be  shown. 


28 


NATURAL  PHILOSOPHY. 


Why  can  there  be  no  such' thing  as  absolute  rest  on  the  earth? 
When  do  we  generally  regard  a body  as  being  at  rest?  What  is 
necessary  in  order  that  a body  may  begin  to  move,  stop  moving,  or 
change  the  direction  of  its  motion  ? 

What  do  you  understand  by  the  inertia  of  matter?  Is  inertia  pos- 
sessed by  bodies  in  motion  as  well  as  by  those  at  rest  ? Show  by  anv 
example  that  this  is  the  case.  What  example  can  you  give  of  a body 
continuing  its  motion  forever? 

How  does  the  amount  of  energy  which  a moving  body  must  lose 
before  it  comes  to  rest,  compare  with  the  amount  required  to  produce 
that  motion  ? Why  can  a person  jump  farther  by  a running  than  by 
a standing  jump  ? 

Describe  any  experiment  by  which  the  inertia  of  matter  may  be 
shown. 

Do  living  bodies  possess  inertia?  What  do  we  find  necessary  to  do 
before  we  can  begin  to  move  from  place  to  place?  Is  any  force  re- 
quired to  stop  our  bodies  when  they  have  once  been  put  in  motion? 

What  do  you  understand  by  friction  ? Give  an  example  of  friction 
acting  as  a resistance  to  motion. 


CHAPTER  III. 

THE  THREE  CONDITIONS  OF  MATTER. 

26.  Solids,  Liquids,  and  Gases.  — Matter  is  found 
in  three  different  conditions,  viz.,  the  solid,  the  liquid, 
or  the  gaseous.  Many  kinds  of  matter  can  be  made  to 
assume  any  of  these  conditions. 

The  molecules  of  a substance,  it  is  believed,  do  not, 
change  in  size  when  the  substance  is  passing  from  a 
solid  to  a liquid  form,  or  from  a liquid  to  a gaseous 
form ; it  is  only  the  distances  which  separate  the  mole- 
cules, and  the  intensity  of  the  forces  that  act  on  them, 
that  change. 

27.  The  Molecular  Forces. — The  molecules  do  not 
touch  one  another,  but  are  separated  by  spaces  or 
pores,  that  are  large  when  compared  Avith  the  size  of 
the  molecules.  The  molecules  are  kept  at  certain  dis- 
tances from  one  another  by  reason  of  forces  which  act 
on  them,  and  Avliich  are  called  the  molecular  forces. 
There  are  two  molecular  forces.  One  tends  to  draw 
the  molecules  together,  and  is  called  the  force  of  mo- 
lecular attraction;  the  other  tends  to  keep  them  apart, 
and  is  called  the  force  of  molecular  repulsion.  The 
molecular  forces,  therefore,  act  in  opposite  directions. 
Molecular  repulsion  is  caused  by  the  action  of  heat. 
The  cause  of  molecular  attraction  is  unknoAvn. 

The  three  conditions  of  mattei\  the  solid,  the  liriuid, 
3*  29 


30 


NA  TURAL  PHILOSO PII Y. 


and  the  gaseous^  result  from  the  different  degrees  with 
which  these  attractive  and  repulsive  fforces  are  exerted 
on  the  molecules. 

28.  Solids. — In  soli'ds,  the  force  of  molecular  attrac- 
tion is  greater  than  that  of  repulsion ; the  molecules, 
therefore,  are  held  together,  and  resist  any  force  tend- 
ing to  separate  them. 

Solids  can  he  fashioned  into  any  form  or  shape,  .since 
the  molecules  are  all  held  together,  and  resist  force 
tending  to  separate  them. 

The  force  of  molecular  attraction  varies  in  different  solids.  A piece 
of  paper,  or  a match  stem,  may  easily  be  pulled  into  smaller  pieces, 
since  the  molecules  are  readily  separated ; but,  if  we  take  a piece  of 
sheet-iron  no  thicker  than  the  pajier,  and  try  to  pull  it  to  pieces,  we 
would  find  that  it  would  require  very  much  greater  force,  because  the 
molecules  of  the  sheet-iron  cling  together  with  far  greater  force  than 
do  those  of  the  paper. 

A steel  wire,  so  thin  as  to  be  almost  invisible  at  the  distance  of 
several  yards,  is  strong  enough  to  hold  a man’s  weight.  In  soft 
wax  or  butter  the  molecules  are  held  together  so  feebly  that  but  little 
force  is  required  to  alter  their  form.  Indeed,  these  substances  are 
very  much  like  liquids. 

29.  Fluids.  — Substances  whose  molecules  move 
freely  over  one  another  are  called  fluids.  There  are 
two  kinds  of  fluid  substances,  viz.,  liquids  and  gases. 

30.  Liquids. — In  liquids,  the  forces  of  molecular 
attraction  and  repulsion  exerted,  are  nearly  equal  to 
each  other.  The  molecules,  therefore,  are  not  all  held 
together,  but  are  very  nearly  independent  of  one  an- 
other, and,  when  acted  on  by  any  force,  can  move  or 
slide  easily  over  one  another. 

From  the  great  freedom  of  motion  of  their  mole- 
cules, liquids  possess  no  definite  shape,  but  at  once 
assume  that  of  the  vessel  in  which  they  are  kept.  A 
quantity  of  water  poured  into  a bottle  will  at  once 


THE  THREE  CONDITIONS  OF  MATTER.  31 


take  the  shape  of  the  inside  of  the  bottle,  and  will 
keep  that  shape  as  long  as  it  continues  in  the  bottle; 
but  let  it  be  poured  into  a cup,  plate,  or  tumbler,  and 
it  will  at  once  take  the  shape  of  the  inside  of  the  cup. 
plate,  or  tumbler. 

31.  Mobile  and  Viscid  Liquids.  — The  force  of 
molecular  attraction  varies  in  different  liquids.  In 
some  the  molecules  are  attracted  to  one  another  with 
much  greater  force  than  in  others,  and  in  them,  there- 
fore, the  molecules  do  not  move  or  flow  over  one 
another  so  readily. 

In  molasses  or  tar,  the  molecules  do  not  move  over 
one  another  as  readily  as  in  alcohol  or  ether,  because 
the  molecules  of  molasses  or  tar  are  held  together 
with  greater  force  than  are  the  molecules  of  the 
alcohol  or  ether. 

Liquids,  like  molasses  or  tar,  in  which  the  molecules 
do  not  flow  or  move  over  one  another  readily,  are 
called  viscid  or  viscous  liquids.  Those  like  alcohol  or 
ether,  in  which  the  molecules  flow  or  move  over  one 
another  readily,  are  called  mobile  liquids. 

AYe  have  seen  that  some  solids,  like  soft  butter  or 
wax,  are  scarcely  to  be  distinguished  from  liquids. 
There  are,  on  the  other  hand,  many  liquids,  like  very 
thick  tar,  which  it  is  difficult  to  distinguish  from 
solids.  The  solid  condition,  indeed,  often  passes  almost 
imperceptibly  into  the  liquid  condition. 

Some  solids,  as,  for  example,  the  metals,  exhibit,  when  subjected  to 
sufficient  pressure,  many  of  the  phenomena  of  flow.  In  the  process 
of  wire  drawing,  a stout  rod  of  cold  iron  or  copper  may  he  drawn 
without  fracture  into  a wire  many  hundred  times  the  length  of  the 
original  rod.  A disc  of  metal,  put  under  a powerful  coining-press,  is 
caused  by  the  pressure  to  flow  into  all  the  cavities  of  the  dies,  thus 
assuming  accurately  their  precise  impressions.  Heavy  blocks  of  ice 


32 


NATURAL  PHILOSOPHY. 


or  stone,  when  subjected  to  long-continued  pressure,  may  be  consider- 
ably bent  or  flexed  without  fracture. 

33.  Gases.  — In  gases,  the  foree  of  repnlsion  is 
stronger  than  that  of  attraction.  Gases,  as  we  have 
seen,  are  one  form  of  fluids,  and  possess  the  ability  to 
flow  to  a much  greater  extent  than  liquids. 

Since  in  gases  the  force  of  repulsion  is  greater  than 
that  of  attraction,  the  molecules  should  be  constantly 
getting  farther  and  farther  apart,  or  the  bulk  of  the 
gas  should  be  constantly  increasing;  but  the  mole- 
cules of  all  bodies  on  our  earth  are  prevented  by 
the  force  of  gravity  from  getting  beyond  a certain  dis- 
tance from  one  another.  In  gases,  therefore,  the  force 
of  gravity,  together  with  that  of  molecular  attraction, 
is  equal  to  the  force  of  repulsion.  Since  the  force 
of  repulsion  is  thus  ever  held  in  check,  it  is  evident 
that  a gas  constantly  tends  to  expand. 

AVe  see  the  influence  of  this  action  in  our  atmos- 
phere. Towards  its  upper  limit  the  air  is  much  lighter 
than  that  nearer  the  earth’s  surface,  since  the  lower 
layers  of  air  have  to  sustain  the  weight  of  those  above 
them,  and  the  particles  are  thereby  packed  more  closely 
together. 

The  increase  in  the  force  of  gravity  near  the  earth’s 
surface  also  causes  a slight  increased  density  of  the 
lower  layers. 

When  a balloon  rises  very  bigb  above  tbe  earth’s  surface,  tbe  gas  it 
contains  expands,  and  occupies  a mucb  greater  bulk  than  it  did  near 
tbe  surface.  If  balloons,  at  tbe  time  of  tbeir  ascent,  are  too  full  of 
gas,  they  often  burst  from  tbis  cause  on  reacbing  moderately  great 
beigbts,  unless  some  arrangement  is  provided  for  tbe  escape  of  the 
excess  of  gas. 

33.  Influence  of  the  Pressure  of  the  Air  on 
Liquids. — AY ere  it  not  for  the  pressure  of  the  air. 


THE  THREE  CONDITIONS  OF  MATTER.  33 


many  bodies  wbich  now  exist  as  liquids  would  be 
gaseous.  W ater,  and  many  other  liquids,  wlien  placed 
iu  an  unlimited  space  from  wbich  all  the  air  has  been 
removed,  will  turn  into  the  gaseous  state.  Gaseous 
bodies  that  are  formed  from  liquids  in  this  way,  or  by 
the  direct  action  of  heat,  are  called  vapors. 

In  many  liquids  the  forces  of  molecular  attraction 
and  repulsion  are  not  in  equilibrium.^  until  the  pressure 
of  the  atmosphere  adds  its  force  to  that-^f  attraction. 

34.  Influence  of  Heat  on  the  Condition  of  Matter. 

— By  the  action  of  heat,  most  solids  become  changed 
into  liquids,  and  most  liquids  into  vapors  or  gases. 
The  action  of  heat  is  to  increase  the  distance  between 
the  molecules,  and  thereby  to  lessen  the  force  of  at- 
traction. 

When  solids  are  changed  into  liquids  by  the  ac- 
tion of  heat,  we  say  that  they  have  been  melted  or 
fused. 

Some  substances  are  very  easily  melted  or  fused.  Ice,  for  example, 
melts  at  quite  a low  temperature.  Butter  softens  when  brought  into 
a warm  room.  In  these  substances  but  a slight  increase  of  tempera- 
ture is  necessary  to  overcome  the  force  of  attraction.  Other  substances 
are  only  melted  or  fused  by  the  action  of  intense  heat.  Cast-iron  re- 
quires the  heat  of  a blast-furnace  in  order  to  melt  it.  Substances  that 
are  very  difficult  to  melt  or  fuse  are  called  refractory  substances. 

When  substances  which  have  been  melted  or  fused 
lose  the  heat  which  caused  them  to  fuse,  they  again 
become  solid.  This  process  is  called  solidification. 
Thus,  water  solidifies  or  freezes  when  it  loses  sufficient 
heat.  Cast-iron  is  obtained  in  any  desired  form  by 
pouring  it,  when  melted,  into  moulds,  in  which  it 
solidifies  on  cooling.  All  liquids  are  changed  into 
solids  if  they  lose  sufficient  heat. 

C 


34 


NATURAL  PniLOSOPHT. 


By  the  action  of  heat,  liquids  become  changed  into 
gases  or  vapors.  This  process  is  called  vaporization. 

On  the  loss  of  the  heat  which  caused  the  vaporiza- 
tion, the  vapor  again  becomes  a liquid.  This  process 
is  called  liquefaction  or  condensation. 

As  a rule,  most  vapors  are  liquefied  by  mere  loss  of  heat.  Ordi- 
nary gases  require  both  loss  of  heat  and  compression  in  order  to  bring 
their  molecules  sufiiciently  near  one  another  to  become  liquids.  Until 
quite  recently,  a number  of  gases  had  never  been  liquefied  by  cold  or 
pressure.  These  were  called  the  incoercihle  gases.  Bj'  subjecting  them, 
however,  to  intense  cold  and  pressure,  they  too  have  been  changed 
into  liquids.  There  are,  therefore,  no  gaseous  substances  that  can 
now  be  considered  as  incoercible. 


oX»io 


Syllabus. 

Matter  exists  in  three  conditions,  viz.,  as  solids,  liquids,  and  gases. 
In  the  same  substance  these  conditions  differ  from  one  another,  not  on 
account  of  the  size  of  the  molecules,  but  on  the  distances  which  sep- 
arate them. 

A group  of  two  or  more  atoms  is  called  a molecule.  The  molecule 
of  a compound  substance  is  the  smallest  quantity  of  that  substance 
that  can  exist. 

The  molecules  are  kept  in  their  relative  positions  by  the  forces  of 
molecular  attraction  and  repulsion.  The  cause  of  molecular  attrac- 
tion is  unknown.  Molecular  repulsion  is  caused  by  heat. 

Solids  are  bodies  in  which  the  force  of  molecular  attraction  is 
greater  than  that  of  repulsion.  Solids  possess  definite  shape,  because 
the  molecules  are  all  held  together,  and  resist  any  force  tending  to 
separate  them. 

The  force  of  molecular  attraction  varies  in  diflferent  solids ; some  are 
easily  pulled  in  pieces,  while  others  can  only  be  broken  by  the  appli- 
cation of  considerable  force. 

Liquids  are  bodies  in  which  the  forces  of  molecular  attraction  and 
repulsion  are  equal  to  each  other.  The  molecules  are  not  all  held 
together,  and  can  move  or  slide  readily  over  one  another. 

Bodies  whose  molecules  move  or  slide  easily  over  one  another 
are  called  fluids.  There  are  two  kinds  of  fluids,  viz.,  liquids  and 
gases. 


QUESTIONS  FOR  REVIEW. 


35 


Liquids  have  no  definite  shape,  but  take  at  once  the  shape  of  the 
vessel  into  which  they  are  poured. 

Liquids  that  flow  readily  are  called  mobile  liquids.  Those  that  do 
not  flow  readily  are  called  viscid  or  viscous  liquids. 

The  solid  condition  passes  almost  imperceptibly  into  the  liquid  con- 
dition. Solid  bodies,  when  subjected  to  great  pressure,  exhibit  many 
of  the  phenomena  of  flow. 

Gases  are  bodies  in  which  the  force  of  attraction  is  greater  than 
that  of  repulsion.  The  pressure  of  the  air  or  the  force  of  gravity, 
however,  makes  the  force  of  attraction  equal  to  that  of  repulsion. 

Gases  are  constantly  tending  to  expand ; their  molecules  possess 
great  freedom  of  motion. 

Heat  causes  solids  to  become  liquids,  and  liquids  to  become  gases. 

When  a solid  has  been  changed  into  a liquid  by  the  action  of  heat, 
we  say  that  it  has  been  melted  or  fused. 

Substances  differ  in  the  amount  of  heat  necessary  to  fuse  them. 
Those  which  are  very  difficult  to  fuse  are  called  refractory  substances. 

When  a substance  which  has  been  melted  is  allowed  to  cool,  it  again 
hardens  or  becomes  solid  : this  is  called  solidification. 

By  vaporization  we  mean  the  change  of  a liquid  into  a vapor  by  the 
action  of  heat.  When  a vapor  is  sufficiently  cooled,  it  again  becomes 
a liquid : the  process  is  called  liquefaction. 

By  the  combined  action  of  cold  and  pressure  all  known  gaseous 
bodies  have  been  changed  into  liquids. 

Questions  for  Review, 

In  what  three  conditions  may  matter  exist? 

Define  atom,  molecule.  What  do  we  understand  by  the  molecule 
of  a compound  substance  ? By  the  molecule  of  an  elementary  sub- 
stance ? Are  the  small  particles  obtained  by  grinding  either  atoms  or 
molecules  ? 

What  are  the  molecular  forces?  Explain  in  full  the  action  they 
exert  upon  the  molecules. 

What  are  solids?  Why  should  solid  bodies  possess  definite  shape? 
Give  any  example  which  shows  that  the  particles  of  different  kinds  of 
solid  substances  are  held  together  with  different  force. 

What  are  fluids  ? What  two  kinds  of  fluid  substances  are  there  ? 
What  are  liquids  ? 

Why  should  liquids  possess  no  definite  shape  ? Have  the  molecules 
of  liquids  no  attraction  for  one  another  ? 


36 


NATURAL  PHILOSOPHY. 


What  is  the  difference  between  mobile  and  viscid  liquids  ? Give  an 
example  of  each.  What  is  the  cause  of  this  difference?  Show  that 
the  solid  state  often  passes  insensibly  into  the  liquid  state. 

What  instances  can  you  give  to  show  that  solids  sometimes  exhibit 
the  phenomena  of  flow  ? 

What  are  gases?  By  what  is  the  force  of  molecular  repulsion  aided 
in  gases  ? Why  should  a gas  be  constantly  endeavoring  to  expand  ? 

Why  is  that  portion  of  our  atmosphere  which  is  near  the  earth's 
surface  denser  than  that  which  is  far  above  it  ? 

What  influence  does  heat  exert  on  the  condition  of  matter?  When 
is  a substance  said  to  be  melted  or  fused  ? What  happens  when  a sub- 
stance which  has  been  melted  or  fused  is  allowed  to  cool  ? 

What  do  you  understand  by  vaporization  ? Under  what  circum- 
stances does  vaporization  occur  ? 

What  is  meant  by  liquefaction  or  condensation  ? In  what  two  ways 
may  liquefaction  or  condensation  be  caused  ? Can  all  gaseous  sub- 
stances be  liquefied  by  the  combined  action  of  cold  and  pressure  ? 


CHAPTER  IV. 

FORCE  AND  MOTION. 

35.  Force  is  anything  which  makes  a body  begin 
to  move,  which  stops  its  motion,  or  Avhich  changes 
the  direction  in  which  it  has  been  moving.  Force  is 
necessary  to  produce  or  modify  motion,  on  account  of 
the  inertia  of  matter. 

All  natural  phenomena  are  caused  hy  force  acting 
upon  matter. 

36.  Varieties  of  Force. — Force  manifests  itself  in 
a variety  of  ways,  and  thus  arise  different  varieties  of 
force. 

We  have  already  briefly  mentioned  the  force  of  gravity,  which 
causes  bodies  to  fall  to  the  ground ; the  force  of  magnetic  attraction, 
which  causes  magnets  to  attract  iron  filings;  the  chemical  force  which 
causes  chemical  changes,  like  the  rusting  of  a pen ; the  muscular  and 
vital  force  which  cause  the  movements  of  living  bodies;  the /orccs  of 
fluid  resistance  and  friction  which  modify  the  motion  of  bodies ; the 
force  of  molecular  attraction,  and  the  force  of  molecular  repulsion  or 
heat  which  keeps  the  molecules  of  matter  at  certain  distances  from 
one  another. 

Besides  these  forces  there  are  a number  of  others,  as,  for  example, 
magnetic  repulsion,  electric  attraction  and  repulsion,  light,  and  elas- 
ticity. 

37.  The  Representation  of  Force.  — In  order  to 
ascertain  the  effect  produced  by  any  force,  we  must 
know 


4 


37 


38 


NATURAL  PHILOSOPHY. 


1st.  The  point  of  application,  or  tlie  point  of  tlie 
body  at  wbich  tbe  force  acts. 

2d.  The  direction  in  which  the  force  acts  ; and, 

3d.  The  intensity  with  which  the  force  acts. 

It  is  convenient  to  represent  forces  by  straight 
arrows ; the  intensity  of  the  force  is  represented  by 
the  length  of  the  arrow;  the  direction  of  the  force  is 
that  in  which  the  arrow  would  fly;  and  its  point  of 
application  is  considered  as  being  situated  at  the  end 
of  the  arrow  which  is  placed  against  the  string  of  the 
bow. 

Thus,  in  the  figure,  we  represent  the  weight  of  the 
body  A,  or  the  force  with  which  it  is 
pulled  down  by  the  attraction  of  the 
earth,  by  the  arrow  y B.  The  amount 
of  this  force  is  represented  by  the  length 
of  the  arrow ; the  force  is  represented  as 
acting  at  the  point  y. 

We  generally  express  the  amount  or 
intensity  of  a force  in  pounds  avoirdupois.  When 
forces  are  represented  by  arrows,  as  in  the  figure,  a 
certain  portion  of  the  length,  as,  for  example,  one 
inch,  is  taken  to  represent  one  pound.  Thus,  if  in 
the  figure  the  arrow  y B were  two  inches  long,  the 
figure  might  represent  a force  of  two  pounds  acting 
at  the  point  y,  in  the  direction  of  y B. 


B 

Pig.  4. 

The  Represen- 
tation of  Force, 


38.  The  Direction  in  which  the  Force  Acts. — 

If  we  take  hold  of  a body  free  to  move  and  pull  it, 
the  body  moves  towards  us ; if  we  push  the  body,  it 
moves  from  us. 

The  direction  in  which  the  force  acts,  therefore,  de- 
termines the  direction  in  which  the  body  moves. 


FORCE  AND  MOTION. 


39 


39.  The  Point  of  Application  of  the  Force. — If  we 

place  the  end  of  a ruler  against  a book  lying  on  a table, 
and  push  the  book,  we  will  notice  that  the  kind  of  mo- 
tion it  acquires  depends  upon  the  part  touched  by  the 
ruler.  If,  for  example,  the  ruler  be  placed  exactly 
against  the  middle  of  one  of  the  ends  of  the  book,  a 
push  will  move  it  straight  forward  in  the  direction 
we  are  pushing;  but  if  the  ruler  be  nearer  one  end 
than  the  other,  although  the  book  will  still  advance 
when  pushed,  it  will  at  the  same  time  move  partly 
around. 

Tlie  point  at  which  the  force  is  applied  determines, 
therefore,  the  nature  of  the  motion. 

40.  The  Intensity  of  the  Force. — If  we  push  the 
book  with  more  force  in  one  instance  than  in  another, 
we  will  find  that  its  motion  will  be  more  rapid  when 
the  greater  force  is  acting  upon  it. 

The  intensity  of  a force,  therefore,  determines  the 
rapidity  of  the  motion. 

41.  Mass  and  Velocity. — If  one  book  is  larger  than 
another,  we  will  find  that  more  force  is  required  to  give' 
it  in  the  same  time  the  same  rapidity  of  motion  as  that 
given  to  the  smaller  book. 

By  the  mass  of  a body  we  mean  the  quantity  of 
matter  it  contains.  In  the  same  substance  the  mass  is 
proportional  to  the  number  of  molecules. 

By  velocity  we  mean  the  distance  through  which  a 
moving  body  passes  in  a given  time.  The  distance  is 
generally  measured  in  feet  and  the  time  in  seconds, 
thus,  by  a velocity  of  six  feet  per  second,  we  mean  a 
velocity  that  will  carry  the  body  through  six  feet  in 
one  second  of  time. 


40 


NATURAL  PHILOSOPHY. 


In  tlie  following  table,  taken  mainly  from  Ganot, 
will  be  found  tbe  velocities  in  feet  per  second  of  a 
number  of  different  objects. 


Table  of  Velocity. 

Snail 

Moderately  slow  river  .... 
Moderately  rapid  river  .... 
Man  walking  ...... 

Quick  military  step 

Moderate  wind 

High  wind 

Hurricane  ....... 

Eailway  train 

Steam-ship  ....... 

Eagle 

Sound  in  air  at  32°  Fah 

Martini-Henry  rifle-hullet  .... 
Point  on  equator  by  rotation  of  earth 
Centre  of  earth  by  revolution  around  the  sun 


i^ee/  per  second, 
b 

TOSTJ 

3 

. 6 

. 3 to  4 
434 
10 
40 

120  to  160 
36  to  75 
22 
100 
. 1090 

. 1330 

. 1520 

. 101000 


42.  Momentum. — We  have  seen  that  tbe  amount 
of  energy  required  to  produce  motion  in  a body 
depends : 

1st.  On  the  mass  of  the  body.  Tbe  greater  tbe  mass, 
tbe  greater  tbe  amount  of  energy  expended  in  produ- 
cing a given  motion,  and 

2d.  On  the  velocity.  In  tbe  same  body,  tbe  greater 
tbe  velocity  tbe  greater  tbe  amount  of  energy  re- 
quired to  produce  it. 

In  order,  tberefore,  to  compare  tbe  total  quantity  of 
motion  possessed  by  one  body  witb  tbat  possessed  by 
another,  or  in  order  to  obtain  tbe  total  quantity  of 
motion  in  any  body,  we  must  multiply  the  mass  of  the 
body  by  its  velocity. 

The  momentum  of  a moving  body.,  or  the  quantity  of 
motion  it  possesses.,  is  equal  to  tbe  mass  of  tbe  body 
multiplied  by  its  velocity. 


FORCE  AND  MOTION. 


41 


If  a given  force  acting  on  a mass  of  ten  pounds  moves  it  at  tlie  rate 
of  ten  feet  per  second,  the  momentum,  or  the  amount  of  energy  of  mo- 
tion, would  be  ten  times  ten,  or  one  hundred ; if  the  same  force  acts  on 
a mass  of  one  pound,  it  must,  in  order  to  give  it  the  same  momentum, 
impart  to  it  a velocity  of  one  hundred  feet  per  second,  since  100  X 1 
= 100.  This  result  would  not  follow,  unless  there  was  no  other  resist- 
ance to  the  motion  of  the  body  than  that  arising  from  its  inertia.  In 
a body  moving  through  air  or  water,  the  increased  fluid  resistance 
would  cause  the  same  force  to  produce  a smaller  velocity. 

43.  Examples  of  Momentum.  — A drop  of  rain 
may  scarcely  bend  the  blade  of  grass  on  which  it  falls  ; 
a moderately  large  hailstone,  falling  with  about  the 
same  velocity,  may  cut  the  leaves  and  branches  from 
trees ; while  a rifle-bullet,  from  its  more  rapid  motion, 
carries  death  in  its  path. 

A floating  chip  is  harmless  when  it  strikes  the  sides 
of  a small  boat,  but  a floating  log  may  crush  the  boat 
if  caught  against  a wharf,  or  other  fixed  obstacle. 
Even  a powerful  ship,  caught  between  two  large  ice- 
bergs moving  in  opposite  directions,  is  as  certainly 
crushed  as  an  egg-shell  when  trodden  on. 

If,  while  in  motion,  we  strike  our  bodies  against 
any  fixed  obstacle,  as  a tree,  the  injury  received  will 
depend  on  the  speed  with  which  we  were  moving.  If 
we  were  walking  at  a moderate  speed,  the  injury 
would  probably  be  but  slight ; if  running,  we  may 
break  a bone ; if  thrown  from  a carriage  while  in 
rapid  motion,  we  may  lose  life. 

44.  Momentum  the  Measure  of  Energy  of  Mo- 
tion. — Since  a body  begins  to  move  solely  from 
energy  expended  on  it,  it  follows  that  the  momentum 
at  any  given  time  represents  the  amount  of  energy 
of  motion  that  the  body  possesses  at  that  time.  The 
momentum.!  therefore!  ^ measure  of  the  amount  of 
energy  of  motion  lohich  a body  possesses. 

4* 


42 


NATURAL  PHILOSOPHY. 


From  the  inertia  of  matter,  it  follows  that  motion 
once  having  been  imparted  to  a body  by  force  acting 
on  it,  the  body  must  continue  to  move  until  it  has  lost 
all  of  its  energy ; if  it  meets  no  resistance,  or  is  acted 
on  by  no  other  force,  it  must  continue  to  move  in  the 
same  direction,  and  with  the  same  velocity  forever. 

45.  Force  not  Affected  by  the  State  of  Rest  or 
Motion.  — A force  acting  on  a body  will  produce  the 
same  effect,  whether  the  body  be  at  rest  or  in  motion. 
Common  experience  proves  this. 

A body  dropped  to  the  floor  of  a rapidl^r-moving 
car  strikes  the  floor  directly  under  the  place  from 
which  it  fell.  The  car-floor  does  not  move  from  under 
the  body  while  it  is  falling,  since  the  body  is  moving 
forward  as  rapidly  as  the  car. 

By  its  rotation,  the  earth  is  rapidly  moving  towards 
the  east;  and  yet  we  find  no  more  difficulty  in  running 
towards  the  west  than  towards  the  east. 

A mounted  acrobat  at  a circus,  in  jumping  through 
a hoop,  does  not  jump  forward;  if  he  did,  he  would  be 
carried  over  the  horse’s  head.  He  simply  jumps  up 
into  the  air,  and,  his  body  having  the  same  velocity 
as  the  horse,  he  falls  again  on  its  back,  just  as  the 
body  dropped  to  the  floor  of  the  car,  strikes  the  floor 
directly  under  the  place  from  which  it  fell. 

Therefore.,  when  two  or  more  forces  act  on  the  same 
body  at  the  same  time,  . each  produces  the  same  effect  as 
if  it  acted  alone. 

46.  Varieties  of  Motion.  — When  a body  moves 
through  equal  distances  in  successive  seconds,  its  mo- 
tion is  said  to  be  uniform. 

When  a body  moves  through  unequal  distances  in 
successive  seconds,  its  motion  is  said  to  be  varied. 


FORCE  AND  MOTION. 


43 


If  the  velocity  of  the  body  is  changed  at  a uniform 
rate,  we  say  that  its  motion  is  uniformly  varied. 

When  the  velocity  regularly  increases,  the  motion 
is  said  to  be  uniformly  accelerated.  When  the  velocity 
regularly  decreases,  the  motion  is  said  to  be  uniformly 
retarded. 

As  gravity  is  constantly  acting  on  a falling  body, 
it  must  have  a uniformly  accelerated  motion,  since 
gravity  is  constantly  adding  to  the  motion  the  body 
has  already  acquired. 

A body  thrown  vertically  upwards  must  have  a 
uniformly  retarded  motion,  since  gravity  is  constantly 
decreasing  its  motion. 

Bodies  whose  motion  is  either  uniform  or  varied, 
may  move  in  straight  lines  or  in  curves,  that  is,  their 
motion  may  be  rectilinear  or  curvilinear. 

Rectilinear  motion  is  that  in  which  the  body  moves 
in  strai2:ht  linos. 

Curvilinear  motion  is  that  in  which  the  body  moves 
in  curved  lines. 

In  rectilinear  motion,  the  body  continues  to  move  in 
the  same  direction.  In  curvilinear  motion,  the  body 
is  constantly  changing  its  direction.  When  a body 
moves  around  a fixed  point,  as,  for  example,  a wheel 
on  an  axis,  the  motion  is  called  rotary. 

4:7.  Components  and  Resultants. — A body  cannot 
move  in  more  than  one  direction  at  the  same  time. 
No  matter,  therefore,  how  many  forces  may  act  at  the 
same  time  on  a body,  their  combined  action  can  only 
produce  motion  in  one  direction;  and  this  motion 
could  have  been  p)roduced  by  a single  force  of  suffi- 
cient intensity  acting  in  the  proper  direction. 

Any  single  force  which  will  produce  the  same  eft'ect 


44 


NATURAL  PHILOSOPHY. 


on  a body,  as  a number  of  separate  forces,  is  called 
their  resultant. 

The  separate  forces  are  called  the  components. 

48.  Direction  of  the  Forces, — When  several  forces 
act  at  the  same  time  on  a body,  they  may  act 

1st.  In  the  same  straight  line. 

2d.  At  some  angle  with  one  another. 

3d.  Parallel  to  one  another. 

49.  Forces  Acting  in  the  same  Straight  Line. — 

When  two  or  more  forces  act  in  the  same  straight  line, 
they  may  act  either  in  the  same  or  in  opposite  direc- 
tions. When  they  act  in  the  same  direction,  the  re- 
sultant is  equal  to  their  sum.  When  two  forces  act 
in  opposite  directions,  the  resultant  is  equal  to  their 
difference;  and  if  the  two  forces  are  of  equal  inten- 
sity, they  will  be  in  equilibrium.^  and  the  body  may 
be  at  rest. 

50.  Forces  Acting  at  some  Angle  with  One 
Another. — When  a body  is- acted  on  by  two  forces  in 
directions  that  form  an  angle  Avith  each  other,  the 
body  Avill  not  move  in  the  direction  of  either  force, 
but  in  some  direction  between  them,  that  is,  the  re- 
sultant will  lie  somewhere  between  the  two  compo- 
nents. 

Thus  suppose, 
for  example,  that 
a man  is  rowing 
a boat  across  a 
riverwith  a force 
that  would  in  a 
certain  time  car- 
ry the  boat  from 

Pig.  5— The  Parallelogram  of  Forces.  ff.  tO  .5  in  the 


FORCE  AND  MOTION. 


45 


direction  of  the  line  A B,  but  that  in  the  same  time, 
by  the  velocity  of  the  stream,  he  would  be  carried 
from  A to  B,  then,  by  the  combined  action  of  these 
two  forces,  he  would  be  carried  to  C along  the  line 
A 0. 

The  direction  and  intensity  of  this  force  are  ascer- 
tained as  follows  : Draw  the  line  B C parallel  to  A D 
and  D C parallel  to  A B.  Then,  if  the  point  (7,  where 
these  two  lines  meet,  be  connected  by  a straight  line 
with  the  point  yl,  this  straight  line  will  give  the 
direction  in  which  the  body  will  move  when  under 
the  action  of  both  forces.  Since  A B and  A D rep- 
resent the  direction  and  intensity  of  the  two  forces, 
the  line  A C will  represent  the  direction  and  inten- 
sity of  the  force  produced  by  their  combined  effect. 

If,  therefore,  we  know  the  lengths  of  A B and  A D, 
we  can  obtain  the  length  of  A C either  by  direct 
measurement  or  by  calculation.  In  this  case  (7  is 
the  resultant,  and  A D and  A B its  components. 

The  figure  A B C D,  being  a four-sided  figure 
bounded  by  parallel  lines,  is  called  a parallelogram, 
and  this  construction  is  generally  called  the  j>aral- 
leloyram  of  forces. 

The  parallelogram  of  forces  affords  another  example  of  the  fact 
already  stated,  that  when  two  or  more  forces  act  on  the  same  body, 
each  produces  the  same  effect  as  if  it  acted  alone.  Thus,  by  the  force 
of  the  man  rowing,  the  boat  would  be  carried  from  Aio  B-,  subse- 
quently, by  the  force  of  the  stream  acting  for  an  equal  time,  it  would 
be  carried  from  B to  C.  We  see  that  each  force  has  produced  the  same 
amount  of  effect  as  if  it  acted  alone,  for,  while  the  man  has  rowed 
the  boat  through  a distance  equal  to  D C,  the  stream  has  carried  it 
through  a distance  equal  to  B C. 

When  two  forces  act  in  any  direction,  forming  an 
angle  with  eacb  other,  the  resultant  is  found  in  the 
same  manner. 


46 


NATURAL  PHILOSOPHY. 


Fig.  6.  — Parallelogram  of  Forces. 


Thus,  suppose  two  forces,  A C and  A B,  act  together 
on  the  point  A,  one  tending  to  produce  motion  along 

the  line  A C,  and  the 
other  along  the  line 
A B ; then,  as  before, 
completing  the  par- 
allelogram A B C D, 
its  diagonal,  A i>, 
represents  the  di- 
rection and  inten- 
sity of  the  resultant. 

Wheu  three  or  more  forces  act  at  the  same  point, 
their  resultant  may  be  obtained  by  first  finding,  as 
before,  the  resultant  of  any  two  of  the  forces ; and 
then  with  this  resultant,  and  any  other  of  the  forces,  a 
second  resultant  is  found,  and  so  on  until  a final  re- 
sultant is  obtained  which  expresses  the  action  of  all 
the  forces. 

Thus,  suppose  three  forces,  A B,  A C,  and  A B,  act 

as  represented  on  the  point 
A.  Completing  the  paral- 
->  lelogram  A B H C.^  its  di- 
agonal A H '\?>  the  resultant 


X 0 


of  the  two  forces  A B and 
A C.  Now  taking  this  re- 
sultant, and  the  third  force 

Fig.  7.-Composition  of  Three  Forces.  completing  the 

parallelogram  A H 0 D, 
its  diagonal,  A 0,  represents  the  resultant  of  the  three 
forces. 


51.  Parallel  Forces. — 'When  two  parallel  forces  act  in  the  same 
direction,  the  resultant  is  always  equal  to  their  sum  ; but  the  point  of 
application  of  the  resultant  does  not  coincide  with  the  points  of  appli- 
cation of  either  of  the  two  forces.  When  the  forces  are  of  equal 
intensity,  it  is  situated  midway  between  them ; but  when  they  are 


FORCE  AND  MOTION. 


47 


Fig.  8,- 


- Parallel  Forces  Acting  in  the  Same 
Direction. 


unequal  in  intensity,  its  distances  from  the  points  of  application  of 
the  two  forces  are  inversely  proportional  to  the  intensities  of  these 
two  forces. 

Thus,  suppose  the  bar  A B,  of  uniform  thickness  and  density,  be 
supported  at  its  centre  C ; it  will  then  be  in  equilibrium,  since,  being 
acted  on  by  two  equal  par-  ^ 

allel  forces,  viz.,  the  weights  aI 
of  the  parts  CB  and  C A, 
the  resultant  of  these  forces 
will  be  found  midway  be- 
tween their  points  of  appli- 
cation, or  at  (7;  and  since 
the  bar  is  supported  at  this  point,  it  will  be  in  equilibrium. 

Suppose,  however,  we  conceive  the  bar  to  be  divided  into  two 
unequal  lengths,  A D and  D B ; then,  since  the  weight  of  .4  i)  is 
greater  than  that  of  D B,  we  have  two  parallel  but  unequal  forces, 
which  may  be  considered  as  acting  at  the  points  E and  F,  the  middle 
points  oi  A D and  D B.  The  point  of  application  of  the  resultant  of 
these  forces  will  not,  therefore,  be  found  midway  between  E and  F, 
but  will  be  at  C,  which  is  as  much  nearer  E than  F,  as  the  force  acting 
at  E is  greater  than  that  acting  at  F,  that  is,  the  distance  of  C from 
E and  F is  inversely  as  the  intensity  of  the  forces  acting  at  E and  F. 
Should  additional  weights  be  hung  to  the  bar  at  the  points  E and  F, 
the  equilibrium  will  not  be  disturbed,  provided  the  weight  at  E is  as 
much  greater  than  the  weight  at  as  the  distance  CF  is  greater 
than  the  distance  C E. 

When  two  parallel  forces  act  in  opposite  directions,  the  point  of 
application  is  never  situated  between  them,  but  is  in  the  extension  of 
a line  joining  the  points  of  application  of  the  two  forces,  on  the  side 
of  the  greater  force,  at  a distance  from  the  points  of  application  of 
the  two  forces  inversely  as  their  intensities.  Thus,  let  two  parallel 
forces,  B C and  A D,  act  at  the  points  B 
and  A,  as  shown.  The  resultant  E F 
takes  the  same  direction  as  that  of  the 
greater  force  B 0;  its  intensity  is  equal 
to  the  difference  of  the  intensity  of  i?  C 
and  A D,  and  its  point  of  application 
E is  so  situated  that  the  distances,  E B 
and  E A,  are  inversely  as  the  forces  B C 
and  A D. 

JJ 

If  two  parallel  forces  of  equal  inten-  Fjg.  g.— Parallel  Forces  Acting  in 
sity  act  in  opposite  directions,  they  can-  Opposite  Directions. 


48 


NATURAL  PHILOSOPHY. 


not  be  replaced  or  held  in  equilibrium  by  a single  force.  These  forces 
form  what  is  called  a couple,  and  tend  to  rotate  the  body  on  which 
they  are  acting. 

52.  Centrifugal  Force. — The  inertia  of  a moving 
body  will  cause  it  to  continue  moving  in  a straight 
line  forever.  To  stop  its  motion  or  to  change  its 
direction,  some  other  force  must  act  on  it.  Since  a 
body  in  circular  motion  is  constantly  changing  its 
direction,  it  is  clear  that  motion  of  this  kind  can 
never  be  produced  by  the  action  of  a single  force, 
because  one  force  would  be  required  to  set  the  body 
in  motion  and  another  to  constantly  act  to  change  the 
direction  of  the  motion  so  produced. 

A stone  tied  to  a string  and  whirled  around,  will 
continue  moving  in  a circle  as  long  as  the  string  is 
held  in  the  hand;  but  if  Ave  let  go  the  string,  the  stone 
will  no  longer  move  in  a circle,  but  will  fly  oft'  in  a 
straight  line,  in  the  same  direction  as  that  in  which 
the  stone  was  moving  at  the  time  the  hand  let  go  the 
string.  The  string  is  constantly  keeping  the  stone  at 
a fixed  distance  from  the  hand,  and  preventing  it  from 
moving  oft'  in  a straight  line.  Since,  however,  the 
stone  neither  moves  towards  the  hand  nor  away  from 
it,  these  two  forces  must  be  equal  to  each  other;  they 
are  not,  however,  directly  opposite,  and  therefore  do 
not  neutralize  each  other,  and  the  body  moves  in  their 
resultant. 

The  force  which  causes  the  body  to  tend  to  move 
oft'  in  a straight  line  in  the  direction  in  which  it  was 
moving  at  any  time,  is  called  the  Centrifugal  Force. 
Centrifugal  force,  or  the  centre-flying  force,  is  merely 
the  result  of  inertia.  The  force  Avhich  prevents  the 
body  from  flying  from  the  centre  is  sometimes  called 
the  Centripetal  Force. 


FORCE  AND  MOTION. 


49 


The  motion  of  the  earth  around  the  sun  aifords  a 
good  instance  of  the  so-called  centrifugal  and  cen- 
tripetal forces.  The  earth,  in  consequence  of  the 
motion  originally  given  to  it,  is  constantly  tending 
to  move  away  from  the  sun.  The  sun,  however,  is 
constantly  attracting  it ; and  these  two  causes  make 
the  earth  move  in  an  almost  circular  path  around  the 
sun. 

53.  Examples  of  Centrifugal  Force.  — Drops  of 
mud  thrown  by  centrifugal  force  from  the  wheels  of 
a carriage  in  rapid  motion  move  off  from  it  in  straight 
lines.  Grindstones  and  the  fly-wheels  of  engines  when 
put  into  very  rapid  rotation  are  sometimes  burst  by 
the  action  of  centrifugal  force.  The  tendency  of  dif- 
ferent portions  of  the  wheel  to  continue  moving  in 
the  direction  in  which  they  were  moving  at  any 
given  moment,  at  last  becomes  stronger  than  the 
cohesion  of  the  particles,  and  the  stone  or  wheel  flies 
into  a number  of  pieces,  which  move  with  consid- 
erable velocity  in  different  directions. 

The  shape  of  the  earth  is  not  that  of  a perfect  sphere.  The  equa- 
torial diameter,  or  the  distance  through  the  centre  at  the  equator, 
is  somewhat  greater  than  the  polar  diameter,  or  the  distance  through 
at  the  poles.  This  bulging  of  the  earth  at  the  equator  was  caused 
by  the  action  of  centrifugal  force. 

Syllabus. 

Force  is  anything  which  makes  a body  begin  to  move,  which  stops 
its  motion,  or  which  changes  the  direction  in  which  it  has  been 
moving. 

All  natural  phenomena  are  caused  by  force  acting  on  matter. 

The  principal  varieties  of  natural  force  are  the  force  of  gravity, 
the  forces  of  magnetic  and  electrical  attraction  and  repulsion,  the 
forces  of  molecular  attraction  and  repulsion,  the  chemical  force,  the 
5 D 


60 


NATURAL  PHILOSOPHY. 


muscular  force,  the  forces  of  fluid  resistance  and  friction,  and  the  forces 
of  light  and  elasticity. 

To  ascertain  the  effect  produced  by  a force  we  must  know,  1st.  The 
point  of  application  of  the  force,  or  the  point  at  which  it  acts.  2d. 
The  direction  in  which  it  acts;  and,  3d.  The  intensity  or  energy 
with  which  it  acts. 

Forces  are  generally  represented  by  arrows:  the  force  acts  in  the 
direction  in  which  the  arrow  flies ; the  intensity  of  the  force  is  repre- 
sented by  the  length  of  the  arrow ; the  point  of  application  is  at  the 
end  of  the  arrow  which  is  placed  against  the  string  of  the  bow. 

The  direction  in  which  the  force  acts  determines  the  direction  in 
which  the  body  moves  ; the  point  at  which  the  force  acts  determines 
the  nature  of  the  motion,  and  the  intensity  of  the  force  determines 
the  velocity  of  the  motion. 

The  mass  of  a body  is  the  quantity  of  matter  it  contains. 

The  velocity  of  a body  is  the  distance  through  which  it  moves  in  a 
unit  of  time. 

The  momentum  of  a body  is  the  quantity  of  motion  it  possesses, 
and  is  equal  to  the  mass  of  the  body  multiplied  by  its  velocity. 

When  we  know  the  momentum  of  a body,  we  can  tell  the  amount 
of  energy  that  has  caused  its  motion. 

A force  acting  on  a body  produces  the  same  effect,  whether  the 
body  is  at  rest  or  in  motion. 

Two  or  more  forces  acting  on  a body  produce  the  same  effect  as  if 
each  force  acted  alone. 

Motion  may  be  either  uniform  or  varied.  Varied  motion  may  be 
either  accelerated  or  retarded. 

Wlien  two  or  more  forces  act  on  a body  at  the  same  time,  they  may 
act,  1st.  In  the  same  straight  line ; 2d.  At  some  angle  with  one  an- 
other ; 3d.  Parallel  to  one  another. 

The  resultant  of  a number  of  forces  is  any  single  force  which  will 
produce  the  same  effect  as  all  the  separate  forces. 

The  separate  forces  are  called  components.  We  can  determine  the 
direction  and  intensity  of  the  resultant  of  several  forces  which  act  in 
directions  at  an  angle  with  one  another,  by  means  of  the  principle  of 
the  parallelogram  of  forces. 

Two  or  more  forces  are  in  equilibrium  when  they  neutralize  each 
other’s  effects,  or  produce  rest. 

The  Centrifugal  force  is  the  force  with  which  a body  moving  in  a 
circular  path  is  constantly  endeavoring  to  move  in  a straight  line. 
Centripetal  force  is  the  force  which  constantly  changes  the  direction 
of  a moving  body,  and  so  causes  it  to  move  in  a curved  path. 


QUESTIONS  FOR  REVIEW. 


51 


Questions  for  Review. 

What  is  force  ? By  what  are  all  natural  phenomena  caused  ? 
Name  the  principal  varieties  of  natural  force. 

What  three  things  are  necessary  in  order  to  ascertain  the  effect  pro- 
duced by  any  force  ? How  are  forces  generally  represented  ? 

What  effect  is  produced  in  the  motion  of  a body  by  the  direction  in 
which  a force  acts  ? What  effect  is  produced  by  the  point  at  which 
the  force  acts  ? What  effect  is  produced  by  the  intensity  of  the  force  ? 

Define  mass,  velocity.  Give  the  velocity  in  feet  per  second  of  any 
four  moving  objects. 

What  do  you  understand  by  the  momentum  of  a body  ? On  what 
two  things  does  the  momentum  of  a body  depend  ? Why  is  the  mo- 
mentum the  real  measure  of  the  amount  of  energy  of  motion  a body 
possesses  ? 

Give  any  examples  of  the  different  effects  produced  by  bodies  whose 
momenta  are  different. 

Is  the  effect  produced  by  a force  acting  on  a body  the  same  when 
the  body  is  at  rest  as  when  it  is  in  motion  ? 

Why  will  a body  dropped  in  a rapidly-moving  car  fall  straight 
down  to  the  floor  ? 

Is  any  more  energy  required  to  move  a body  towards  the  west  than 
towards  the  east  ? 

When  two  or  more  forces  act  on  a body  at  the  same  time,  will  each 
produce  a similar  effect  as  if  it  acted  alone  ? Illustrate  by  examples. 

What  is  the  difference  between  uniform  and  varied  motion?  What 
two  kinds  of  varied  motion  are  there? 

Define  rectilinear  motion,  curvilinear  motion,  rotary  motion. 

When  are  forces  said  to  be  in  equilibrium  ? 

What  do  you  understand  by  the  resultant  of  several  forces  ? What 
is  meant  by  the  components  of  a force  ? 

When  several  forces  act  on  a body  at  the  same  time,  in  what  dif- 
ferent directions  may  they  act  ? 

When  several  forces  act  on  a body  in  the  same  straight  line,  how 
is  their  resultant  found  ? 

Describe  the  principle  of  the  parallelogram  of  forces.  How  do  we 
determine  the  resultant  of  several  forces  acting  on  the  same  body  in 
directions  parallel  to  one  another  ? 

What  do  you  understand  by  centrifugal  force  ? By  centripetal 
force  ? Could  a single  force  cause  motion  in  a circle  ? 


CHAPTER  V. 

THE  MECHANICAL  POWERS. 

54.  Machines. — A machine  is  any  arrangement  of 
parts  by  means  of  which  force  is  so  transmitted  from 
one  point  to  another  that  its  intensity  or  direction,  or 
both,  are  modified.  The  force  Avhich  is  used  in  mov- 
ing a machine  is  called  the  power.  The  resistance  to 
be  overcome,  or  the  work  to  be  done,  is  called  the 
wo7'Jc,  weight.1  or  load. 

A pair  of  scissors  affords  a good  example  of  a simple 

machine.  Here  the  power, 
which  is  the  strength  of  the 
fingers,  is  applied  at  the 
handles,  in  a direction  which 
causes  the  handles  to  come 
together.  The  work  accomplished  by  the  scissors  is 
the  cutting  of  some  material  placed  between  the  two 
blades. 

55.  The  Principle  of  Velocities. — When  there  is 
no  other  resistance  to  the  motion  of  a body  than  that 
arising  from  its  inertia,  a given  amount  of  force  acting 
for  a certain  time  on  a given  mass  will  impart  to  it 
a given  velocity ; but  the  same  force,  acting  for  the 
same  time  on  another  mass  twice  as  great,  will  impart 
to  it  but  half  this  velocity. 


Fig.  10.“  A Simple  MacMne. 


52 


THE  MECHANICAL  POWERS. 


53 


If  a straiglit  rod,  A i?,  supported  on  a fixed  point,  F.^ 
nearer  A tlian  B,  be 
moved  so  as  to  as- 
sume tire  position 
shown  by  the  line 
A'  B',  then,  since  the 
end  B moves  over  Fig.  ll.— The  Principle  of  Velocities, 
the  curved  path  B B'  in  the  same  time  that  the  end 
A moves  over  the  curved  path  A J.',  the  velocity  of 
B must  be  as  much  greater  as  the  velocity  of  A as 
B B'  is  greater  than  A A'. 

But  B B'  is  as  much  greater  than  A A'  as  the  line 
F B w greater  than  the  line  FA.  If  then  FB  be 
twice  as  great  as  F A,  the  velocity  of  the  end  B will 
be  twice  that  of  the  end  A. 

If  now  a weight  of  one  pound  be  hung  at  the  end 
5,  then,  disregarding  the  weight  of  the  bar  A B,  the 
pound  at  B will  exert  a force  capable  of  lifting  as 
much  more  than  one  pound  at  A^  as  the  velocity  of  B 
is  greater  than  the  velocity  of  A ; and,  since  in  this  case 
the  velocity  of  B is  twice  that  of  A,  one  pound  at  B 
will  lift  two  pounds  at  A.  A,  however,  will  move 
with  a velocity  but  half  as  great  as  that  with  which 
B moves;  for,  since  B is  acting  on  a mass  twice  as 
great  as  its  own,  it  can  in  the  same  time  impart  to  A 
but  half  the  velocity. 

The  rod,  A B,  moving  about  the  point,  F,  is  in  fact 
a simple  machine.  Power  applied  at  either  end  will 
overcome  a resistance  at  the  other  end,  and  the  power 
and  weight  will  be  in  equilibrium,  or,  in  other  words, 
the  power  will  be  able  to  balance  the  weight  when  the 
power  multiplied  by  the  distance  through  which  it  moves 
is  equal  to  the  weight  multiplied  by  the  distance  through 
which  it  moves. 

5* 


54 


NATURAL  PHILOSOPHY. 


The  bar  will  then  be  in  equilibrium  because  the 
momentum  at  either  end  of  the  bar  will  be  equal  to 
that  at  the  other  end.  Thus,  at  ^ a mass  of  one  is 
moving  with  a velocity  of  two,  and  the  momentum  is 
therefore  equal  to  1 x 2 = 2,  while  at  a mass  of  two 
is  moving  with  a velocity  of  1,  and  the  momentum  is 
therefore  equal  to  2 x 1=2. 

This  principle  will  enable  us  to  determine  the  re- 
lation existing  between  the  power  or  force  used  in 
driving  any  machine  and  the  work  done  by  the 
machine ; /or,  disregarding  friction  and  fluid  resist- 
ances, we  have  only  to  notice  the  distance  through  which 
the  power  must  he  moved  in  order  to  cause  a given 
movement  of  the  weight.  Suppose,  for  example,  in  any 
machine  that,  in  order  to  move  the  weight  through 
one  foot,  the  power  moves  through  ten  feet,  then  a 
power  of  one  will  move  a weight  of  ten. 

Disregarding  the  weight  of  the  rod  A B,  and  supposing,  as  before, 
the  distance  A F to  be  one-half  of  F B,  then  we  can  see,  from  what 
has  been  said  about  parallel  forces,  that  if  one  pound  be  hung  at  B 
and  two  pounds  at  A,  they  will  balance  each  other ; since  the  two  par- 
allel forces  of  different  intensities,  acting  in  the  same  direction,  are  in 
equilibrium  when  their  resultant  is  at  F,  wdiich  is  as  much  farther 
from  B than  from  A as  the  force  at  B is  less  tlran  that  at  A. 

56.  No  Energy  Gained  by  Machines. — It  must  not 
be  supposed,  from  the  example  just  given,  that  a ma- 
chine creates  energy.  No  more  work  can  be  done  bv 
any  machine  than  that  which  has  been  expended  in 
moving  it.  In  fact,  from  the  resistances  which  all 
machines  offer  to  motion,  we  never  get  as  much  effec- 
tive work  out  of  a machine  as  that  Avhich  has  been 
put  into  it.  All  that  any  machine  can  do  is  to  change 
the  amount  of  resistance  that  can  he  overcome,  by  chang- 
ing the  time  in  tuhich  it  is  done. 


THE  MECHANICAL  POWERS. 


55 


Suppose,  in  Fig.  11,  F B were  ten  times  the  length 
of  F A.!  then  one  pound  at  B could  move  ten  pounds 
at  A.  Here  it  might  seem  as  if  energy  sufficient  to 
raise  nine  pounds  had  been  created ; but  this  is  not  so, 
since  the  single  pound  moves  through  ten  times  the 
distance  as  that  through  which  the  ten  pounds  are 
raised,  or  .exerts  its  force  through  ten  times  the  space. 

To  make  this  clearer,  suppose  that,  by  exerting  his  entire  strength,  a 
man  could  just  lift  one  hundred  pounds.  Then,  by  the  u.se  of  the 
simple  machine  above  described,  he  could  raise  10  X 100  = 1000  lbs. ; 
but  to  raise  these  thousand  pounds  through  one  foot,  he  would  be 
required  to  continue  to  exert  a force  of  one  hundred  pounds  through 
ten  feet ; and  this  would  clearly  be  the  same  as  if  he  divided  the 
thousand  pounds  into  ten  separate  parcels  of  one  hundred  pounds 
each,  and  for  ten  successive  times  exerted  his  strength  of  one  hundred 
pounds  through  one  single  foot. 

Though  machines  gain  nothing  in  energy,  yet  their  gain  in  con- 
venience is  often  considerable.  In  the  case  of  the  simple  machine  just 
described,  there  is  a saving  in  the  time  that  would  otherwise  be  lost  in 
passing  from  parcel  to  parcel ; besides  which,  one  man  is  enabled  to 
raise  weights  without  dividing  them,  which  it  would  otherwise  be  im- 
possible for  him  to  raise. 

In  general,  whatever  be  the  nature  of  the  work  for  which  the 
machine  is  designed,  its  perfection  depends  on  the  convenience  it  se- 
cures in  the  performance  of  that  work. 

57.  Mechanical  Powers.  Simple  Machines. — Ho 

matter  how  complicated  any  piece  of  machinery,  it 
is  made  up  of  various  combinations  of  a number  of 
simple  machines.  These  simple  machines  are  some- 
times called  the  mechanical  powers. 

The  mechanical  povjers  are  the  lever.^  the  wheel  and 
axle.^  the  pulley the  inclined  plane,  the  wedye,  and  the 
screw. 

The  six  mechanical  powers  just  named  are  in  re- 
ality modifications  of  the  lever  and  of  the  inclined 
plane. 

In  all  that  will  be  said  about  simple  machines,  it  is 


56 


NATURAL  PHILOSOFIIY. 


F 


w 

Fig.  12.  — Lever  of  the  1st  Class. 


comes 


w 


supposed  that  no  force  is  lost  during  its  transmission 
from  one  part  of  the  machine  to  another. 

58.  The  Lever. — 1.  The  lever  consists  of  an  inflex- 

^ ihle  rod  or  bar  moving  about  a 

fixed  point,  called  the  fulcrum. 

A force  or  power  applied 
at  one  part  of  the  lever  over- 
a resistance  or  lifts  a weight  at  another  part.- 
-P  The  difierent  forms  of  levers 
may  be  arranged  in  three  class- 
es, according  to  the  relative 
positions  of  the  fulcrum,  the 
power,  and  the  weight. 

In  the  first  class,  the  fulcrum  is  between  the  power 
P and  the  weight ; in  the  second 

class,  the  weight  is  between 
-- .'■'■j  the  fulcrum  and  the  power; 

w and  in  the  third  class,  the 
Fig.  14.  — Lever  of  the  3d  Class.  . . , , , « . 

power  IS  between  the  luicrum 

and  the  weight. 

Examples  of  levers  of  the  first  class  are  found  in  a 
pair  of  scissors  ; in  a crowbar,  when  used  to  raise 
blocks  of  stone ; in  the  common  balance  for  weighing, 
and  in  a pair  of  pincers. 

Examples  of  levers  of  the  second  class  are  found  in 
nut-crackers,  where  the  nut  to  be  cracked,  is  placed 

between  the  fulcrum,  E, 


Fig.  13.  — Lever  of  the  2d  Class. 


and  the  part,  P,  where 
the  power  is  applied.  A 
Fig.  15. -Levers  of  the  2d  Class.  opened  bv  a 

hand  applied  to  the  knob  is  another  example : the 
weight  of  the  door  is  between  the  hinges,  which  act 
as  the  fulcrum,  and  the  knob  where  the  power  is  ap- 
plied. A Avheelbarrow  is  another  example. 


THE  MECHANICAL  POWERS. 


57 


Examples  of  levers  of  the  third  class  are  found  in 
tlie  sugar-tongs,  tlie  fire- 
tongs,  and  in  the  com- 
mon foot-treadle. 


The  Arms  of  the  Le- 
ver.— 2.  The  shortest 
distance  from  the  ful- 

crum  to  the  direction  in  which  the  power  acts,  is 
called  the  arm  of  the  power;  the  shortest  distance  from 
the  fulcrum  to  the  direction  in  which  the  Aveight  acts, 
is  called  the  arm  of  the  weight.  These  two  distances 
are  called  the  arms  of  the  lever. 

When  the  f 

power  acts,  as  / \ 

in  Fig.  17,  in  a 

direction,  B P.  ^ \ ^ 

at  right  angles 
to  the  length  of 

O 

the  lever,  then ' 

FB  the  arm 
of  the  power ; 
but  if  it  acts  in  any  oblique  direction,  as  B P',  then 
F B',  the  shortest  distance  from  the  fulcrum  to  the 
direction,  B'  P',  of  the  power,  is  the  arm  of  the  poAver. 


V 

p 

Fig.  17.—  Tlie  Arms  of  a Lever. 


P 


The  Effects  of  the  Lever. — 3.  The  effect  produced 
by  the  poAver  in  any  lever  depends  on  the  relative 
lengths  of  the  arm  of  the  poAver  and  the  arm  of  the 
Aveight ; for,  as  the  distances  through  which  the  poAver 
and  the  Aveight  move  are  directly  as  the  arms  of  the 
poAver  and  of  the  Aveight,  the  power,  multiplied  by 
the  arm  of  the  poAver,  is  equal  to  the  Aveight  multi- 
plied by  the  arm  of  the  weight. 


58 


NATURAL  PHILOSOPUY. 


In  Figs.  12,  13,  and  14,  a bar  of  the  same  length  is  shown,  em- 
ployed as  a lever  of  the  first,  second,  and  third  classes  respectively. 
In  Fig.  12,  if  F P,  the  arm  of  the  power,  is  three  times  as  great  as 
F W,  the  arm  of  the  weight,  then  a force  of  one  pound  acting  at  P 
will  balance  a weight  of  three  pounds  at  IF. 

In  Fig.  13,  by  placing  the  power  and  fulcrum  at  the  extremities  of 
the  lever,  the  distance,  F IF,  the  arm  of  the  weight,  may  be  made  one- 
fourth  that  of  F P,  the  arm  of  the  power,  so  that  a force  of  one  pound 
at  P will  balance  four  pounds  at  IF.  The  same  bar,  therefore,  when 
used  as  a lever  of  the  second  class,  can  bo  employed  to  overcome  a 
greater  resistance  than  when  used  as  a lever  of  the  first  class. 

In  Fig.  14,  if  F P he  one-fourth  as  great  as  F IF,  then  a force  of  one 
pound  acting  at  P will  balance  but  one-fourth  of  a pound  at  IF.-  it 
will  move  it,  however,  with  a velocity  four  times  greater  than  its  own. 
In  levers  of  the  third  class,  the  velocity  of  the  weight  is  always 
greater  than  the  velocity  of  the  power. 

59.  The  Wheel  and  Axle. — In  the  tvlieel  and  axle, 

the  power  applied  at 
the  circumference  of  a 
wheel  is  employed  to 
raise  a weight  attached 
to  a rope  wound  around 
an  axle.  The  figure 
shows  a form  of  the 
wheel  and  axle  called 
Pig.  18— The  Windlass.  windlass.  The 

power  is  applied  to  either  a wheel  or  winch  at  Tl',  and 
raises  a weight,  if,  attached  by  a rope  to  the  axle,  -4. 
The  wheel  and  axle  is  simply  a modification  of  the 
lever : the  fulcrum  is  at  the  axis ; the  weight  is  ap- 
plied at  one  side  of  the  axle,  while  the  power  is  a]3- 
plied  at  some  point  on  the  wheel. 

Since,  by  one  complete  turn  of  the  wheel,  the  weight 
is  only  raised  the  length  of  the  rope  wound  once 
around  the  axle,  a force  of  ojte  pound  applied  at  the 
wheel  will  raise  a iceight  of  as  many  more  pounds  hung 
to  the  axle  as  the  circumference  of  the  wheel  is  greater 


THE  MECHANICAL  POWERS. 


59 


than  the  circumference  of  the  axle.  TVater-Avlieels  and 
steering-apparatus  for  vessels  employ  the  principle  of 
the  wheel  and  axle. 

60.  The  Pulley,  — The  pulley  consists  of  a wheel 
turning  on  its  axis,  and  having  an  edge  over  which  a 
flexible  band  or  rope  passes.  Pulleys  may  be  fixed  or 
movable.  In  the  fixed  pulley,  no  advantage  is  gained 
except  the  change  in  the  direction  of  the  motion,  since,  if 
the  rope  is  pulled  down  one  foot  by  the  power,  it  will 
raise  the  weight  through  an  equal  dis- 
tance. In  the  movable  pulley,  the  block 
or  frame.  A,  to  which  the  weight  is  at- 
tached, is  movable.  If  the  power,  P, 
moves  downwards  through  two  feet, 
the  weight,  IF,  would  be  raised  through 
but  one  foot,  since  the  rope  is  pulled 
from  both  C and  D.  In  such  a pul- 
ley, therefore,  a force  of  one  pound  at 
P would  raise  a weight  of  two  pounds 
at  IF.  The  fixed  pulley  is  a continuous 

lever  of  the  first  class  with  equal  arms. 

The  lever  includes  both  the  wheel  and  axle  and  the 
'pulley. 

61.  The  Inclined  Plane.  — Instead  of  raising  a 
weight  directly  through  a given  height,  it  may  be 
raised  gradually  through  the  same  height  by  moving 
it  up  an  inclined  plane.  In  this  way  heavy  casks  or 
barrels  are  moved  out  of  deep  cellars. 

The  efficiency  of  the  inclined  plane  depends  on  the 
direction  in  which  the  power  is  applied.  We  will  con- 
sider but  two  cases,  viz. : 1st,  when  the  power  is  applied 
in  the  direction  of  the  length  of  the  plane ; 2d,  ichen 
it  is  applied  in  the  direction  of  the  base  of  the  plane. 


60 


NATURAL  PEILOSOPHY. 


An  example  of  the  first  case  is  seen  in  the  figure, 
where,  to  raise  the  barrel  through  a height  equal  to 

the  height  of  the  plane, 
we  must  roll  the  barrel 
over  the  whole  length  of 
the  plane.  If  the  lenfjth 
of  an  inclined  plane  he  four 
times  as  yreat  as  its  heifjht, 
a force  of  one  pjov.nd  will 
he  able  to  'move  a weight 

of  four  pounds  up  the  plane. 

In  the  second  case,  in  order  to  raise  the  weight 
through  the  distance  of  the  height,  the  power  moves 
through  a distance  equal  to  the  base  of  the  plane ; if, 
then,  the  base  of  an  inclined  plane  he  three  times  as 
great  as  its  height,  a force  of  one  pjound  would  he  able 
to  ptoll  three  pounds  up  the  plane. 

62.  The  Wedge.  — The  wedge  is  a modified  form 
of  inclined  plane,  in  which,  instead  of 
moving  the  weight  up  the  jfiane,  the 
plane  is  moved  under  the  weight.  The 
Avedge  is  used  when  great  force  is  to  be 
exerted  in  a small  space,  such,  for  ex- 
ample, in  splitting  Avood  or  stone,  or  in 

Pig.  21.  The  Wedge.  pj-0gsiijg  opg  or  juices  from  seeds.  The 
edges  of  such  cutting  tools  as  scissors  kniAms,  chisels, 
hatchets,  and  razors  are  forms  of  Avedges. 

63.  The  Screw. — The  scrcAV  is  another  modifica- 
tion of  the  inclined  plane,  and  has  the  same  relation  to 
a simple  plane  that  a spiral  staircase  has  to  a straight 
one.  If  a piece  of  paper  be  cut  in  the  form  of  a 
right-angled  triangle  and  Avrapped  around  a pencil,  as 
shown  in  the  cut,  the  edge  of  the  paper,  Avhich  cor- 


THE  MECHANICAL  POWERS. 


61 


responds  to  the  length  of  the  plane,  will  form  a spiral 
line  around  the  pencil  in  a direction 
which  will  be  the  same  as  that  of  the 
thread  of  a screw.  As  the  power  which 
turns  the  screw  moves  it  through  one 
complete  turn,  the  screw,  and  anything 
against  which  it  is  pushing,  as,  for  exam- 


pie,  the  movable  plate  of  a copying-press,  22,— The  Prin- 

Fig.  23,  advances  through  the  distance 
between  any  two  consecutive  threads.  If  the  power 
acting-  on  a screw  move  through 
a circumference  of  24  inches, 
and  the  distance  between  any 
two  consecutive  threads  be  yo  of 
an  inch,  a power  of  one  jDound 
applied  at  the  head  of  the  screw 
w'ould  move  a weight  of  240 


pounds  at  the  other  end,  since  23.— A Copying-Press, 
the  power  would  move  through  a distance  240  times 
as  great  as  that  through  which  the  weight  moves. 

The  wedye  and  screw  are  modifications  of  the  inclined 
plane. 

64.  Perpetual  Motion.  — Many  foolish  men,  igno- 
rant of  the  fact  that  machines  do  not  create  energy,  but 
only  transmit  the  energy  imparted  to  them,  have  art- 
fully endeavored  so  to  design  a machine  that,  when 
it  has  once  been  set  into  motion,  will  not  only  con- 
tinue to  move  forever  afterwards,  but  would  even  give 
motion  to  other  things,  Avithout  losing  any  of  its  own 
motion. 

If  a body  could  be  set  in  motion  where  it  would  be 
exposed  to  no  friction  or  fluid  resistances,  it  would  then 
continue  to  move  forever.  Such  conditions,  hoAA'ever, 
are  impossible  on  the  earth ; and  even  if  they  did  exist, 


62 


NATURAL  PHILOSOPHY. 


a macTiine  so  set  in  motion  would  be  useless,  since  it 
would  possess  only  a certain  quantity  of  energy, 
namely,  that  applied  to  start  it ; and  if  it  should  be 
used  to  give  motion  to  other  machines,  or  to  do  any 
work,  it  would  cease  moving  as  soon  as  it  had  expended 
an  amount  of  energy  equal  to  that  originally  imparted 
to  it. 

Syllabus. 

A machine  is  any  arrangement  of  parts  by  means  of  which  power  is 
so  transmitted  from  one  point  to  another  that  its  intensity  or  direction, 
or  both,  are  modified. 

The  force  causing  the  motion  of  a machine  is  called  the  power ; the 
resistance  to  be  overcome  is  called  the  work  or  load. 

If  a certain  force  acting  on  a body  having  a certain  mass  produce  a 
given  velocity,  the  same  force  acting  for  the  same  time  on  a body  with 
twice  the  mass  will  produce  but  one-half  as  great  a velocity;  or  double 
the  force  acting  on  a given  mass,  will  produce  twice  as  great  a velocity 
as  half  the  force  acting  on  the  same  mass. 

If  a straight  rod  be  moved  while  unequally  balanced,  the  ends  will 
pass  through  unequal  spaces  in  the  same  time,  and  therefore  possess  un- 
equal velocities. 

If  weights  be  hung  at  the  extremities  of  such  a rod.  they  must  be 
unequal,  in  order  to  produce  equilibrium  or  to  balance  the  rod.  If  a 
given  force  acts  at  the  longer  end  of  such  a rod,  it  will  exert  at  the 
shorter  end  a force  as  much  greater  as  its  own,  as  the  velocity  with 
which  it  moves  is  greater  than  the  velocity  with  which  the  other  end 
moves. 

If  unequal  weights  be  balanced  at  the  extremities  of  a bar  supported 
at  a point  nearer  one  end  of  the  bar  than  the  other,  either  weight 
multiplied  by  its  distance  from  the  point  of  support  will  be  equal  to 
the  other  weight  multiplied  by  its  distance  from  the  point  of  support ; 
or,  if  the  bar  be  moved  about  the  point  of  support,  either  weight  mul- 
tiplied by  the  distance  through  which  it  moves  will  be  equal  to  the 
other  weight  multiplied  by  the  distance  through  which  it  moves. 

No  machine  creates  energy ; it  merely  changes  the  amount  of  work 
that  can  be  done  by  changing  the  time  in  which  it  is  done. 

All  machines,  however  complicated,  are  made  up  of  various  combi- 
nations of  a few  simple  machines,  called  the  mechanical  powers. 


SYLLABUS. 


63 


The  mechanical  powers  are  six,  viz.,  the  lever,  the  wheel  and  axle, 
the  pulley,  the  inclined  plane,  the  wedge,  and  the  screw.  These  are 
all  modifications  of  the  lever  or  of  the  inclined  plane. 

The  wheel  and  axle,  and  the  pulley  are  modifications  of  the  lever ; 
the  wedge  and  the  screw  are  modifications  of  the  inclined  plane. 

A lever  is  a straight  rod  or  bar  moving  freely  about  a point  called 
the  fulcrum.  There  are  three  classes  of  levers  ; in  the  first,  the  fulcrum 
is  between  the  power  and  the  weight ; in  the  second,  the  weight  is 
between  the  power  and  the  fulcrum  ; in  the  third,  the  power  is  between 
the  weight  and  the  fulcrum. 

Scissors,  pincers,  the  common  balance,  and  the  crow-bar  when  used 
to  lift  weights,  are  examples  of  levers  of  the  first  class. 

Nut-crackers,  a wheelbarrow,  and  a door  opened  by  a hand  at  the 
handle,  are  examples  of  levers  of  the  second  class. 

Sugar-tongs,  fire-tongs,  and  the  common  foot-treadle,  are  examples 
of  levers  of  the  third  class. 

In  all  levers,  the  arm  of  the  power  is  the  shortest  distance  between 
the  fulcrum  and  the  direction  of  the  power ; the  arm  of  the  weight  is 
the  shortest  distance  between  the  fulcrum  and  the  direction  of  the 
weight.  The  power  and  weight  will  always  balance  each  other  when 
the  power  multiplied  by  the  arm  of  the  power  is  equal  to  the  weight 
multiplied  by  the  arm  of  the  weight. 

In  the  wheel  and  axle,  a force  of  one  pound  applied  at  the  circum- 
ference of  the  wheel  will  raise  as  many  more  pounds  of  weight  hung 
to  the  axle  as  the  circumference  of  the  wheel  is  greater  than  the  cir- 
cumference of  the  axle. 

Pulleys  are  fixed  or  movable : in  fixed  pulleys,  nothing  is  gained 
except  a change  of  direction ; in  the  movable  pulley,  the  power  can 
raise  a weight  as  much  greater  than  its  own  as  the  distance  through 
which  it  moves  is  greater  than  the  distance  through  which  it  raises 
the  weight. 

In  the  inclined  plane,  if  the  power  is  applied  parallel  to  the  length 
of  the  plane,  it  will  exert  a force  as  much  greater  than  its  own  as  the 
length  of  the  plane  is  greater  than  the  height  of  the  plane ; but  if  the 
force  is  applied  parallel  to  the  base  of  the  plane,  it  will  exert  a force 
as  much  greater  than  its  own  as  the  base  of  the  plane  is  greater  than 
the  height. 

The  wedge  is  used  wherever  great  force  is  to  be  exerted  in  a small 
space. 

Any  force  acting  to  turn  a screw  will  cause  the  screw  to  move  with 
a force  as  much  greater  than  its  own,  as  the  circumference  of  the  circle 
through  which  the  screw  moves  is  greater  than  the  distance  between 
any  two  contiguous  threads. 


64 


NATURAL  PHILOSOPHY. 


Perpetual  motion  is  impracticable : 1st,  Because  all  moving  bodies 
on  the  earth  encounter  resistances ; 'and,  2d,  Because,  even  if  a body 
could  move  without  friction  and  fluid  resistance,  it  would  cease  moving 
as  soon  as  it  had  expended  in  giving  motion  to  other  bodies  an  amount 
of  energy  equal  to  that  originally  imparted  to  itself. 

Questions  for  Review. 

Define  machine,  power,  weight  or  work. 

A straight  bar  is  supported  so  as  to  move  freely  about  a point  nearer 
one  end  of  the  bar  than  the  other;  when  will  weights  Lung  at  the 
ends  of  the  bar  be  in  equilibrium  ? 

1 f no  energy  is  gained  by  a machine,  why  is  it  that,  when  properly 
using  a lever,  we  can,  by  exerting  a force  of  only  one  hundred  pounds, 
move  a weight  of  one  thousand  pounds  ? 

Name  some  of  the  advantages  gained  by  the  use  of  machines? 
What  do  you  understand  by  a simple  machine  or  mechanical  power  ? 

Name  the  mechanical  powers.  Under  what  two  heads  may  all  the 
mechanical  powers  be  arranged  ? Name  the  powers  which  are  included 
under  each  of  these  heads. 

What  is  a lever  ? What  are  the  arms  of  a lever  ? Into  what  three 
classes  may  all  levers  be  arranged  ? What  are  the  relative  positions 
of  the  fulcrum,  power,  and  weight  in  each  of  these  classes  ? 

Name  some  levers  of  the  first  class  ? of  the  second  class  ? of  the  third 
class  ? In  which  class  is.  each  of  the  following,  viz.,  a foot-treadle, 
scissors,  sugar-tongs,  a door  opened  by  a hand  applied  at  the  knob,  a 
wheelbarrow,  a balance,  and  a pair  of  pincers  ? 

Describe  the  wheel  and  axle.  Why  will  a certain  weight  hung  at 
the  circumference  of  the  wheel  lift  a greater  weight  hung  at  the  cir- 
cumference of  the  axle  ? 

What  two  kinds  of  pulleys  are  there  ? What  is  the  nature  of  the 
advantage  gained  by  the  fixed  pulley?  By  the  movable  pulle}'? 

If  two  inclined  planes  have  the  same  height,  but  one  is  twice  as 
long  as  the  other,  how  much  less  force  would  be  required  to  roll  the 
same  barrel  up  the  longer  plane  than  up  the  shorter  one  ? Why  ? 

For  what  is  the  wedge  generally  used  ? Give  some  examples  of  wedges. 

Show  that  the  screw  is  a modified  form  of  inclined  plane.  How  can 
the  power  of  a screw  be  estimated  ? If  the  power  acting  on  a screw 
move  through  a circumference  of  four  inches,  and  the  distance  between 
any  two  contiguous  threads  be  j’jj  of  an  inch,  with  how  much  force  would 
the  screw  advance  when  a force  of  one  pound  was  applied  at  the  head? 


CHAPTER  VI. 

GRAVITATION. 

65.  Effect  of  Gravity. — We  have  seen  that  unsup- 
ported bodies  fall  to  the  earth  because  they  are  attracted 
towards  it  by  the  force  of  gravity. 

The  real  nature  of  gravity  is  not  thoroughly  under- 
stood, but,  from  repeated  observations  and  experiments, 
it  is  believed  that  every  particle  of  matter  in  the  uni- 
verse exerts  an  attractive  force  for  every  other  particle 
of  matter,  and  that  this  attractive  force  would  eventu- 
ally bring  all  matter  to  one  place  were  it  not  for  op- 
posing causes. 

By  the  force  of  gravity^  we  mean  the  attractive  force 
that  one  mass  of  matter  exerts  upon  another.  The 
weight  of  a body  is  caused  by  the  earth’s  attraction  for 
the  body. 

66.  English  and  French  Systems  of  Weight. — The  unit  of  weight 
both  in  this  country  and  in  England  is  the  pound.  Unfortunately, 
the  pound  is  of  two  distinct  kinds,  viz.,  the  pound  avoirdupois  and  the 
pound  troy.  The  grains  are  alike  in  both  of  these  pounds,  but  the 
other  subdivisions  are  different.  The  avoirdupois  pound  contains  7000 
grains,  and  the  troy  pound  contains  5760  grains ; in  the  former  there 
are  16  ounces,  and  in  the  latter  12  ounces. 

In  France,  the  unit  of  weight  is  the  gramme  and  its  multiples  and 
subdivisions.  The  gramme  is  the  weight  of  a cubic  centimetre  of  water 
at  the  temperature  of  its  greatest  weight,  viz.,  at  39°.2  Fahr.,  and  is 
ec[ual  to  about  15.432  English  grains.  The  following  table  gives  the 
names  and  values  of  these  multiples  and  subdivisions. 

6*  E 


65 


66 


NATURAL  PHILOSOPHY. 

Grammes.  Grains. 

1 Kilogramme 

= 1000 

15432.34 

1 Hectogramme 

= 100 

1543.23 

1 Decagramme 

= 10 

154.32 

1 Gramme 

= 1 

= 

15.43 

1 Decigramme 

= 0.1 

1.543 

1 Centigramme 

= 0.01 

.154 

1 Milligramme 

= 0.001 

.0154 

. Law  of  Universal  Gravitation.- 

—The  law 

universal  gravitation  was  discovered  bj  Sir  Isaac 
Newton,  an  English  philosopher,  after  a long  series 
of  observations  and  calculations.  It  may  be  stated 
as  follows,  viz. : 

Every  particle  of  matter  in  the  universe  attracts  every 
other  particle  of  matter  with  a force  that  is  directly  pro- 
portional to  the  mass,  and  inversely  proportional  to  the 
square  of  the  distance. 

One  thing  is  directly  proportional  to  another  wlien  it  increases  or 
decreases  in  the  same  ratio  that  the  other  increases  or  decreases. 
Thus,  when  we  say  that  the  force  of  attraction  is  directly  proportional 
to  the  mass,  we  mean  that,  if  the  mass  of  a hodj’  were  doubled,  the 
attraction  it  would  exert  for  other  bodies  would  also  he  doubled ; or 
if  its  mass  were  trebled,  its  attractive  force  would  be  trebled. 

One  thing  is  inversely  ptroporiioival  to  another  when  it  increases  or 
decreases  in  the  same  ratio  that  the  other  decreases  or  increc^es. 
Thus,  if  one  is  made  twice  as  great,  the  other  becomes  but  half  as 
great.  When  we  say,  for  e-^ample,  that  every  particle  of  matter 
attracts  every  other  particle  with  a force  that  is  inversely  propor- 
tional to  the  square  of  the  distance,  we  mean  that  if  at  a certain 
distance  the  attraction  between  two  bodies  was  represented  by  one, 
then,  if  this  distance  were  made  twice  as  great,  the  attraction  would 
be  but  one-fourth,  viz.,  inversely  as  the  square  of  the  distance. 

68.  Attraction  Proportional  to  the  Mass.  — The 
greater  the  mass  the  greater  the  attraction.  An  un- 
supported body  falls  to  the  earth  because  the  earth 
attracts  it ; the  body  also  attracts  the  earth,  and  al- 
though the  pull  which  the  earth  has  for  the  body  is 


GRAVITATION. 


67 


no  greater  in  amount  than  that  which  the  body  has  for 
the  earth,  yet  the  quantity  of  matter  in  the  earth  is 
so  much  greater  than  that  in  the  body,  that  the  motion 
imparted  to  the  earth  is  as  much  less  than  that  imparted 
to  the  body  as  the  mass  of  the  earth  is  greater  than 
the  mass  of  the  body ; hence,  although  the  earth  rises 
to  meet  the  falling  body,  its  motion  is  imperceptible. 

69.  Attraction  Inversely  Proportional  to  the 
Square  of  the  Distance.  — The  farther  a body  is 
carried  above  the  earth’s  surface  the  less  the  attrac- 
tion, and,  since  the  weight  of  a body  is  due  to  the 
earth’s  attraction  for  it,  the  less  the  weight.  A body 
on  the  earth’s  surface  is  approximately  four  thousand 
miles  from  the  centre  of  the  earth.  If  a pound  weight 
be  carried  four  thousand  miles  above  the  earth’s  sur- 
face, it  would  weigh  but  one  quarter  of  a pound,  since, 
as  its  distance  from  the  earth’s  centre  is  doubled,  the 
earth’s  attraction  for  it  is  diminished  to  one-fourth. 

Since  the  earth  is  bulged  out  at  the  equator,  a body 
when  at  the  equator  is  farther  from  the  earth’s  centre 
than  when  at  the  poles.  The  same  body,  therefore, 
would  weigh  more  at  the  poles  than  at  the  equator ; 
191  pounds  at  the  equator  would  weigh  195  pounds  at 
the  poles. 

If,  however,  we  take  a body  below  the  surface  of  the  earth,  it 
would  weigh  less  than  at  the  surface,  since  that  part  of  the  earth 
that  is  above  it,  would  pull  in  the  opposite  direction  to  that  part  of  the 
earth  below  it,  and  would  therefore  decrease  its  weight.  At  the  centre 
of  the  earth,  a body  would  have  no  weight,  since  the  attraction  would 
be  the  same  in  all  directions,  that  is,  the  body  would  be  pulled  as 
much  in  one  direction  as  in  another. 

The  combined  effect  of  mass  and  distance  on  the  amount  of  attrac- 
tion, is  seen  in  the  tides  of  the  ocean,  which  are  caused  by  the 
attraction  of  the  sun  and  moon.  The  mass  of  the  sun  is  very  much 
greater  than  that  of  the  moon,  but  since  the  moon  is  so  much  nearer 


G8 


NATURAL  PH ILOSO PH Y. 


the  earth  than  the  sun,  the  influence  which  the  moon  exerts  in  causing 
the  tides  is  greater  than  that  exerted  by  the  sun. 

In  order  to  understand  the  efiects  of  gravity,  tve 
must,  as  with  any  other  force,  ascertain,  1st.  TJie  direc- 
iion  in  which  it  acts  ; 2d.  Its  point  of  application,  and 
dd.  Its  intensity. 


70.  The  Direction  of  Gravity.  — Gravity  acts  in 
a vertical  direction.  A vertical  line  is  one  tvhich  is 
perpendicular,  or  at  right  angles  to  a horizontal  line. 
A horizontal  line  is  one  at  right  angles  to  a vertical 


line,  or  is  one  which  extends  in  the  same  direction  as 
a limited  water  surface. 

The  direction  of  gravity  can  be  ascertained  by  the 
use  of  the  plumbdine,  Avhich  con- 
sists of  a weight,  W,  attached  to 
the  end  of  a string.  If  the  other 
end  of  the  string  be  held  in  the 
hand  when  the  weight  has  come 
to  rest,  the  string  will  be  stretched 
ill  a vertical  direction,  and  the 
plumb-line  will  point  directly  to- 
wards the  eentre  of  the  earth. 
As  it  is  the  string  which  pre- 
vents the  weight  from  falling  to 
the  earth,  and  the  string  comes 
to  rest  in  a vertical  position,  the 
direction  of  the  vertical  must  be  that  in  which  gravity 
acts  on  the  body. 


w 


fig.  24. — The  Plumh-Line. 


71.  The  Point  of  Application.  Centre  of  Grav- 
ity. — As  gravity  acts  alike  on  all  the  particles  of 
matter  in  a body,  there  must  be  as  many  separate 
points  of  application  as  there  are  particles.  The 


GRA  VITATION. 


69 


direction  in  wlricli  gravity  acts  on  eacli  of  tliese  parti- 
cles is  vertically  downwards. 

The  separate  pulls,  which  may  be  regarded  as  so 
many  separate  components,  may  all  be  replaced  by  a 
single  resultant,  which,  since  they  all  act  in  the  same 
direction,  is  equal  to  their  sum.  This  resultant  is 
equal  to  the  weight  of  the  body,  and  its  point  of  appli- 
cation is  called  the  centre  of  gravity^  because  it  is  a point 
at  which  the  whole  weight  of  the 
body  may  be  regarded  as  collected. 

In  the  figure,  the  vertical  dotted 
lines,  a,  J,  c,  d,  e,/,  etc.,  represent  the 
separate  pulls  which  gravity  exerts 
on  the  particles  of  a body.  G A 
represents  their  resultant,  and  G,  the 
point  of  application  of  this  result- 
ant, is  the  centre  of  gravity  of  the 
body. 


Fig.  25.— The  Centre  of 
Gravity. 


72.  Method  of  Determining  the  Centre  of  Grav- 
ity.— The  centre  of  gravity  of  any  body  may  be  de- 
termined as  follows : Suspend  the  body  by  a string 
attached  at  any  part,  and  allow  the  body  to  come  to 
rest.  Notice  the  direction  in  which  the  string,  if  ex- 
tended downwards,  would  pass  through  the  body.  The 
centre  of  gravity  will  be  situated  somewhere  in  this 
line.  Attach  the  string  to  some  other  part  of  the 
body,  and  again  suspend  it,  and  observe  as  before  the 
direction  of  the  line  extended  downwards  from  the 
string  when  the  body  is  at  rest.  The  centre  of  grav- 
ity of  the  body  will  be  situated  somewhere  in  this 
line.  As  the  centre  of  gravity  is  a point,  a point 
situated  in  two  lines  must  be  at  their  intersection. 
The  point,  therefore,  where  these  two  lines  intersect 
each  other  will  be  the  centre  of  gravity  of  the  body. 


70 


NATURAL  PHILOSOPHY. 


Experiment. — In  a rectangular  plate  of  tin,  bore  small  holes  at  a,  &, 
and  c,  as  shown  in  Fig.  26.  Tie  a string  to  the  plate  at  h,  and  sus- 


pend the  plate  by  the  string,  and  when 
it  lias  come  to  rest,  draw  the  line  & d 
in  the  direction  of  the  string  extended. 

Then  attach  the  string  at  some  other 
point,  as  c,  Fig.  27,  and,  proceeding  as 
before,  draw  the  line  c e.  The  centre  of  gravity  of  the  plate  will  be 
found  at  g.  where  the  line  h d is  cut  by  the  line  c e. 

Find  in  the  same  way  the  centre  of  gravity  of  any  other  body. 


Fig.  27.—  Method  of  Finding  the 
Centre  of  Gravity. 


73.  A Body  Supported  at  its  Centre  of  Gravity 
will  be  at  Rest, — Since,  in  Fig.  26,  tlie  plate  of  tin 
comes  to  rest  in  the  position  sliotvn,  tbe  shaded  por- 
tion, h c d /,  must  be  of  the  same  weight  as  the  un- 
shaded portion,  aide-,  for  were  one  of  these  por- 
tions heavier  than  the  other,  it  would  fall,  and  the 
line  h d would  take  some  other  direction.  So  also  in 
Fig.  27,  the  shaded  portion,  c e or,  is  of  the  same 
weight  as  the  unshaded  portion,  c e f.  The  same  is 
true  of  any  other  position  of  the  body : the  part  of 
the  body  on  one  side  of  the  line  of  direction  is  always 
equal  in  Aveight  to  tbe  part  on  the  opposite  side.  J. 
body  at  rest  siqyported  at  its  centre  of  gravity  tcill  there- 
fore remain  at  rest,  since  the  weight  is  then  equally  dis- 
tributed about  the  point  of  support. 


GRA  VITA  TION. 


71 


74.  Equilibrium  of  Bodies  Supported  on  an  Axis. 

— A body  supported  ou  an  axis,  around  wbicli  it  is  free 
to  turn,  will  be  in  equilibrium  only,  wlien  the  point  of 
support  and  the  centre  of  gravity  are  in  the  same  ver- 
tical line. 

The  point  of  support  may  be  in  three  different  posi- 
tions, viz.,  1st.  Above  the  centre  of  gravity ; Below 
the  centre  of  gravity  ; and,  3d.  At  the  centre  of  gravity. 
These  three  positions  correspond  to  three  different 
kinds  of  equilibrium,  viz.,  stable,  unstable,  and  neu- 
tral equilibrium. 

75.  Stable  Equilibrium.  — In  stable  equilibrium, 
the  point  of  support  is  above  the  centre  of  gravity. 
Any  motion  of  the  body  around  the  point  of  support 
raises  the  centre  of  gravity,  and  the  body  when  left  to 
itself  again  assumes  a position  of  stable  equilibrium. 

If  a disc  be  cut  from  a flat  piece  of  card-board,  and 
a hole  be  made  at  S by  a large 
needle,  so  as  to  allow  the  card  to 
move  freely  around  the  needle, 
and  the  card  held  as  shown  in  Fig. 

28,  it  will  come  to  rest  in  a po- 
sition of  stable  equilibrium,  since  rinm. 

the  point  of  support,  /S',  is  above  the  centre  of  grav- 
ity, 6^,  and  in  the  same  vertical  line  with  it. 

76.  Unstable  Equilibrium. — In  unstable  equilib- 
rium, the  point  of  support  is  below  the  centre  of 
gravity.  Any  motion  of  the  body  causes  the  centre 
of  gravity  to  fall : the  body  does  not  afterwards  tend 
to  assume  its  old  position  of  equilibrium,  but  assumes 
one  of  stable  equilibrium. 

If  the  pasteboard  disc  be  held  as  shown  in  Fig. 

29,  it  will  be  in  a position  of  unstable  equilibrium. 


Fiff.  28.— StaMe  Equilit- 


72 


NATURAL  PH ILOSOPHY. 


since  tlie  point  of  support,  is  below  the  centre  of 
gravity,  and  in  the  same  ver- 
tical line  Avith  it.  If  the  disc  be 
slightly  moved,  the  centre  of  grav- 
ity falls,  and  the  disc  assumes  the 

Fig.  29.— UnstaUe  Equilib-  . . , . , i j.  .c 

rinin,  position  shown  in  the  last  hgure. 

77.  Neutral  Equilibrium. — In  neutral  equilibrium, 
the  point  of  support  coincides  with  the  centre  of  grav- 
ity, and  the  body  will  remain  at  rest  in  whatever  posi- 
tion it  may  be  moved  to  about  the 
axis. 

If  the  pasteboard  disc  be  held 
as  in  Fig.  30,  it  will  be  in  a posi- 
Fig.  30.— Neutral  Eqnilib-  tion  of  neutral  equilibrium,  since 
the  point  of  support  is  at  the  cen- 
tre of  gravity,  and  no  matter  how  the  disc  be  turned, 
it  will  remain  at  rest. 

78.  Equilibrium  of  Bodies  Resting  on  a Flat  Sur- 
face.— When  a body  has  more  than  one  point  of  sup- 
port, as,  for  example,  when  some  portion  of  the  bodv 
is  resting  on  a fiat  surface,  it  is  not  necessarj*  that  the 
centre  of  gravity  be  above  any  one  of  these  points  of 
support  in  order  that  the  body  may  be  in  equilibrium. 
It  is  sufficient  if  the  vertical  line,  passing  through  the 
centre  of  gravity,  passes  within  the  base  on  which  the 
body  rests.  The  equilibrium  may  be  stable,  unstable, 
or  neutral. 

When  the  centre  of  gravity  is  as  loiv  as  it  can  get, 
the  body  is  in  stable  equilibrium.  The  loAver  the  cen- 
tre of  gravity,  and  the  greater  the  base  on  Avhicli  the 
body  rests,  the  more  stable  the  equilibrium.  When 
the  relative  positions  of  the  eentre  of  gravity  and  the 


GRA  VITATION. 


73 


Fig.  31.—  Stable  Equilib- 
rinm. 


Fig.  32.— Stable  Eqnilib- 
linnij  but  less  Stable 
than  at  A. 


point  of  support  remain  the  same  in  any  position  of 
a body,  it  is  in  neutral  equilibrium. 

A book  placed  as  at  A,  Fig.  31, 
is  in  stable  equilibrium,  since  it  is 
resting  on  a large  base,  and  its  cen- 
tre of  gravity  is  low. 

When  standing  as  at  i?.  Fig.  32, 
it  is  still  in  stable  equilibrium;  but 
since  the  base  on  which  it  rests  is  smaller  than  when 
placed  as  at  4,  and  its  centre  of 
gravity  is  higher,  the  equilibrium 
is  less  stable  at  B than  at  A. 

When  placed  as  at  (7,  Fig.  33,  the 
equilibrium,  though  still  stable,  is 
less  stable  than  at  since  the  base 
on  which  it  rests  is  still  smaller,  and 
the  centre  of  gravity  higher. 

If  the  book  could  be  rested  on  the  corner  of  one  of 
its  covers,  it  would  be  in  a position  of 
unstable  equilibrium.  If  it  could  be 
rested  on  the  edge  of  one  of  its  covers, 
it  would  still  be  in  unstable  equilibrium  ; 
though  the  equilibrium  would  be  less  un- 
stable than  in  the  preceding  case,  since, 
in  the  latter  case,  it  would  have  a num- 
ber of  points  of  support,  while  in  the 

former  it  would  have  but  one.  less  Stable  than 

8/t  B 

A sphere  resting  on  a level  table  is 
in  neutral  equilibrium,  because  no  movement  of  the 
body  can  change  the  relative  positions  of  the  centre 
of  gravity  and  the  point  of  support. 

A wagon  loaded  with  brieks  is  in  more  stable  equi- 
librium than  one  loaded  with  hay,  because  the  centre 
of  gravity  is  nearer  the  ground. 


74 


NATURAL  PHILOSOPHY. 


In  a boat  loaded  with  people,  the  equilibrium  is 
more  stable  if  the  passengers  remain  seated.  When 
they  stand  up,  the  centre  of  gravity  is  raised,  and  the 
boat  if  small  may  upset. 


Kxperiment.- 

d 


Fig.  34.— Experiment 
in  Stable  Equilib- 
rium. 


A stout  pin,  a,  Fig.  34,  is  stuck  upright  in  a cork 
placed  in  the  mouth  of  a narrow  bottle,  c.  The 
blunt  end  of  a stout  needle,  e,  is  stuck  in  a cork, 
d,  on  the  sides  of  which  a number  of  penknives,/ 
and  g,  are  placed,  as  shown  in  the  figure.  If  the 
point  of  the  needle  be  now  carefully  placed  on  the 
head  of  the  pin,  it  will  be  found  that  it  will  rest 
there  in  a position  of  moderately  stable  equilib- 
rium, and  the  cork  and  knives  can  even  be  moved 
around  without  falling. 

Caution. — As  the  head  of  a pin  is  more  or  less 
rounded,  it  may  be  rubbed  flat  with  a file. 


79.  The  Laws  of  Falling  Bodies. — The  laws  of 
falling  bodies  were  discovered  by  Galileo,  an  Italian 
philosopher.  They  may  be  expressed  briefly  as  fol- 
lows, viz. ; 

1.  First  Law. — The  velocity  of  a falling  body  is  not 
affected  hy  its  mass. 

Gravity  acts  on  each  of  the  atoms  of  a body ; if  the 
atoms  were  all  separated  from  one  another,  each  would 
fall  to  the  earth  with  the  same  velocity,  because  the 
same  force  acts  on  each.  Their  being  united  in  one 
mass  makes  no  difference,  since  gravity  acts  on  each  as 
though  it  were  alone,  therefore  the  number  of  atoms 
in  any  body,  or  its  mass,  has  no  effect  on  its  velocity. 

Should  we  yoke  two  equally  fast  horses  abreast  of  each  other,  so  as 
to  give  them  perfect  freedom  of  motion,  the  two  together  would  not  be 
able  to  run  any  faster  than  either  separately.  It  is  the  same  with  the 
motion  of  atoms  towards  the  earth. 

2.  Second  Law.  — The  velocity  of  a falling  body  is 
not  affected  by  the  shape  or  the  nature  of  the  body. 


GRA  VITATION. 


75 


So  far  as  our  experience  goes,  tliis  law  would  appear 
to  be  incorrect,  since  a piece  of  gold  in  the  shape  of  a 
ball,  will  fall  more  rapidly  through  the 
air  than  when  beaten  out  into  gold-leaf; 
again,  a small  piece  of  cork  falls  less 
rapidly  through  the  air  than  a piece  of 
iron  of  the  same  size. 

It  is  the  resistance  of  the  air  which 
causes  these  apparent  exceptions  to  the 
law.  In  a vacuum,  or  empty  space,  all 
bodies,  whatever  their  size  or  material, 
fall  with  the  same  velocity  ; a feather  and 
a leaden  bullet  let  fall  from  the  same 
height,  at  the  same  time,  would  reach  the 
bottom  of  the  empty  vessel,  Fig.  35,  at 
the  same  instant. 

Even  in  the  air,  two  iron  weights,  one 
weighing,  say,  an  ounce,  and  the  other  a 
pound,  if  allowed  to  fall  from  the  hand  at 
the  same  time,  will  reach  the  floor  in  so 
nearly  the  same  time  that  the  eye  is  una- 
ble to  detect  any  difference. 

3.  Third  Law. — The  velocity  acquired  hy  a falling 
body  at  the  end  of  any  given  time  is  proportional  to  the 
times.1  and  is  as  the  numbers  1,  2,  3,  4,  etc. 

Thus,  in  a body  falling  freely  from  a state  of  rest, 
the  velocity  at  the  end  of  the  first  second  is  about  32 
ft.  per  sec. ; the  velocity  at  the  end  of  the  second  sec- 
ond is  2 X 32  = 64  ft. ; the  velocity  at  the  end  of  the 
third  second  is  3 x 32  = 96  ft. 

4.  Fourth  Law.  — The  distances  fallen  through  in 
successive  times  increase  as  the  odd  numbers  1,  3,  6,  7, 
etc. 

A body  falling  freely  from  a state  of  rest  passes 


Fig.  35.— Bodies 
Faliingthrongli 
anEmptySpace. 


76 


NATURAL  PHILOSOPHY. 


tlirouD-li  about  sixteen  feet  durincr  tlie  first  second  of 

O O 

its  descent.  But  tlie  velocity  of  falling  bodies  is  con- 
stantly increasing,  since  gravity  is  constantly  giving 
to  the  body  a fresh  impulse,  vrhich  is  added  to  the 
velocity  already  acquired. 

At  the  beginning  of  the  first  second  of  its  fall,  the  velocity  of  the 
body  is  equal  to  nothing,  since  it  starts  from  a state  of  rest.  From 
the  moment  of  falling,  the  velocity  regularly  increases.  Its  mean  or 
average  velocity  during  any  second  would  therefore  be  equal  to  the  mean 
between  the  velocity  at  the  beginning  and  that  at  the  end  of  that  sec- 
ond. Thus,  the  mean  velocity  during  the  first  second  = — ^ — = 10; 
or  the  final  velocity  at  the  end  of  the  first  second  is  32 /h 

At  the  beginning  of  the  second  second,  the  body  has  acquired  a ve- 
locity that,  if  gravity  ceased  to  act,  would  carry  it  during  the  second 
second  through  32  ft.  But  during  the  second  second,  gravity  would 
carry  it  over  an  additional  distance  of  16  ft.,  and  therefore,  during 
the  second  second,  the  body  will  fall  through  32+  16  = 48  ft.  But 
48  = 3X16.  The  body  during  the  second  second  falls  through  three 
times  the  distance  that  it  fell  during  the  first  second. 

At  the  beginning  of  the  third  second,  which  is  of  course  the  same 
as  the  end  of  the  second  second,  the  final  velocity  is  according  to 
the  third  law,  2 X 32  ft.  = 64  ft.  During  this  second,  gravity,  as  be- 
fore, carries  it  an  additional  distance  of  16  ft.,  and  the  distance  fallen 
through  equals  64  + 16  = 80  ft.  But  80  = 5 X 16-  The  body,  during 
the  third  second  of  its  fall,  passes  through  a distance  five  times  as  great 
as  during  the  first  second. 

5.  Fifth  Law. — The  total  spaces  pjassed  throucjli  are 
proportional  to  the  squares  of  the  times. 

During  the  first  second,  the  body  falls  through  16  ft ; during  the 
second  second,  it  falls  through  48  ft. ; at  the  end  of  the  second  sec- 
ond, it  has  fallen  through  a total  distance  of  48  + 16  = 64  ft.  But 
64  is  four  times  as  great  as  16,  that  is,  at  end  of  the  second  second 
the  body  has  fallen  through  a space  of  2 X 2.  or  2 squared,  greater  than 
what  it  fell  during  the  first  second,  or,  in  other  words,  the  whole  space 
is  proportional  to  the  square  of  the  time. 

During  the  tliird  second,  the  body  falls  through  80  ft. ; at  the  end 
of  the  third  second,  the  bod}'  has  fallen  through  a total  distance  of 
16  + 48  + 80  = 144.  But  144  = 9 X 16,  and  9 = 3X3,  or,  as  before, 
the  total  distance  is  proportional  to  the  square  of  the  time. 


GRA  VITA  TION. 


'll 


80.  The  Pendulum.  — A pendulum  consists  of  a 

body,  5,  suspended  by  a string  or  rod,  , 
a 5,  from  a fixed  support,  a,  on  wbicli  ^ 
it  is  free  to  move.  The  body,  i,  is  / 
called  the  hob  of  the  pendulum.  j 
When  the  pendulum  is  at  rest,  it  / 
assumes  the  position  of  the  vertical,  / 
a h.  If,  now,  the  bob  be  raised,  so  / 
as  to  assume  the  position  shown  at  a j 
c,  it  will,  when  allowed  to  fall,  move  / 
towards  its  old  position,  a h,  along  the  / 
curved  line  c h.  When,  however,  the  *3- — i 
position  a b is  reached,  the  pendulum  30,— The  Pendu- 

does  not  cease  moving ; the  momen- 

turn  it  has  acquired  in  falling  from  c to  ^ will  carry  it 
past  this  position  to  d.  When  it  reaches  c/,  it  again 
falls  towards  6,  and  acquires  momentum  sufficient  to 
carry  it  to  c,  and  so  on,  the  pendulum  continuing  to 
swing  between  c and  d.  Each  complete  swing  from  c 
to  d^  or  from  d to  c,  is  called  a7i  oscillation.  The  time 
it  takes  the  pendulum  to  move  through  each  complete 
swing  is  called  the  tmie  or  duration  of  an  oscillation. 
The  curved  line  6 c,  or  i (/,  which  marks  the  distance 
the  pendulum  has  been  moved  from  the  vertical  a b,  is 
called  the  amplitude  of  the  oscillation. 

81.  The  Laws  of  the  Pendulum. — Fi7-st  Law.  In 

the  same  pendulum.,  if  the  amplitude  of  the  oscillation 
is  not  very  great.,  the  time  of  oscillation  for  different 
a7nplitudes  is  nearly  the  same. 

Unless  the  pendulum  is  connected  with  a spring- 
er weight,  the  resistance  which  the  air  offers  to  its 
movement  will  cause  it  to  swinu;  through  smaller 
and  smaller  arcs.  When,  however,  these  arcs  are  not 
7* 


a 


78 


NATURAL  PHILOSOPUY. 


very  large,  the  time  required  to  move  through  the 
longer  arc  will  be  the  same  as  that  required  to  move 
through  any  shorter  one. 

Second  Law. — In  ]}endulums  of  different  lengths.^  the 
duration  of  an  oscillation  is  proportional  to  the  square 
roots  of  the  lengths. 

That  is,  the  longer  the  pendulum  the  slower  its 
oscillation.  If  the  lengths  of  two  pendulums  are  as  1 
and  9,  the  duration  of  their  oscillations  will  be  as  v'T 
and  1/9,  or  as  1 is  to  3,  that  is,  a pendulum  nine  times 
the  length  of  another,  will  move  three  times  more 
slowly,  or  the  time  of  its  oscillation  will  be  three 
times  as  great. 

82.  The  Intensity  of  Gravity. — The  intensity  or 
energy  with  which  gravity  acts  ou  any  given  mass  of 
matter,  can  be  ascertained  by  the  weight  of  that  mass. 
This,  however,  varies  in  different  latitudes,  being,  as 
we  have  seen,  greater  at  the  poles  than  at  the  equator. 

83.  Intensity  of  Gravity  and  the  Pendulum. — 

Since  gravity  is  the  cause  of  the  motion  of  the  pendu- 
lum, Ave  can  determine  the  variations  in  the  intensity 
of  gravity  in  different  parts  of  the  earth  by  noticing 
the  time  of  oscillation  of  the  pendulum.  If  Ave  car- 
ried the  same  pendulum  from  the  equator  to  the  poles, 
Ave  Avould  find  that  its  oscillations  would  become  grad- 
ually more  and  more  rapid,  thus  showing  that  the 
force  of  gravity  Avas  becoming  greater  and  greater. 

Syllabus. 

The  force  of  gravity  is  caused  by  the  attractive  force  which  par- 
ticles of  matter  exert  on  one  another.  Every  particle  of  matter  in 
existence  attracts  every  other  particle  with  a force  that  is  directly  pro- 


SYLLABUS. 


79 


portional  to  its  mass,  and  inversely  proportional  to  the  square  of  the 
distance  between  them. 

The  mass  of  the  earth  is  so  great  that  our  experience  of  the  force  of 
gravity  is  confined  almost  entirely  to  a force  tending  to  draw  bodies 
down  towards  the  earth’s  centre. 

If  one  body  has  twice  the  mass  of  another,  its  attractive  force  will 
be  twice  as  great.  If  two  bodies  are  twice  as  far  apart  at  one  time  as 
at  another,  their  attractive  force  at  the  greater  distance  will  be  four 
times  less  than  at  the  smaller  distance. 

The  force  of  gravity  acts  in  a vertical  direction,  and  tends  to  pull 
bodies  towards  the  centre  of  the  earth.  This  direction  can  be  shown 
by  means  of  a plumb-line. 

The  point  of  application  of  gravity  is  situated  at  the  centre  of  grav- 
ity, or  the  point  in  a body,  at  which  the  resultant  of  all  the  pulls 
which  the  earth  exerts  on  the  particles  of  the  body,  acts.  The  weight 
of  a body  may  be  considered  as  concentrated  at  the  centre  of  gravity. 

A body  supported  at  its  centre  of  gravity  wiU  he  in  equilibrium, 
because  its  weight  is  evenly  distributed  around  this  point. 

There  are  three  kinds  of  equilibrium,  viz.,  stable,  unstable,  and  neu- 
tral. A body  supported  on  an  axis,  around  which  it  is  free  to  move, 
will  not  be  in  equilibrium  unless  the  point  of  support  and  the  cen- 
tre of  gravity  are  in  the  same  vertical  line.  When  the  point  of  sup- 
port is  above  the  centre  of  gravity,  the  equilibrium  is  stable  ; if  at  the 
centre  of  gravity,  the  equilibrium  is  neutral ; if  below  the  centre  of 
gravity,  the  equilibrium  is  unstable. 

In  bodies  resting  on  a level  surface,  the  equilibrium  is  most  stable, 
if  the  centre  of  gravity  is  in  the  lowest  position  possible  ; if  the  centre 
of  gravity  is  not  as  low  as  it  can  be,  the  equilibrium  may  be  stable  or 
unstable;  if  the  relative  positions  of  the  centre  of  gravity  and  the 
points  of  support  are  not  altered  by  any  movement  of  the  body,  the 
equilibrium  is  neutral. 

The  broader  the  base  of  a body,  and  the  lower  the  centre  of  gravity, 
the  more  stable  the  equilibrium. 

The  velocity  of  a falling  body  is  independent  of  its  mass,  and  is  not 
affected  by  the  nature  or  shape  of  the  body. 

The  velocity  acquired  by  a falling  body  at  the  end  of  any  given 
time,  is  proportional  to  the  time. 

The  distances  fallen  through  in  successive  times  increase  as  the  odd 
numbers  1,  3,  5,  7,  etc. 

The  total  spaces  fallen  through  are  proportional  to  the  squares  of 
the  times. 

In  the  same  pendulum,  if  the  amplitude  of  the  oscillation  is  not 


80 


NATURAL  PHILOSOPHY. 


very  great,  the  time  of  oscillation  for  different  amplitudes  is  nearly 
the  same.  In  pendulums  of  different  lengths,  the  duration  of  an  oscil- 
lation is  profiortional  to  the  square  root  of  the  length. 

The  intensity  of  gravity  at  the  surface  of  the  earth  is  greater  at  the 
poles  than  at  the  equator,  because  at  the  poles  a body  is  nearer  the 
earth’s  centre  than  when  at  the  equator. 

The  intensity  of  gravity  can  lie  determined  by  the  use  of  the  pen- 
dulum. The  same  pendulum  will  oscillate  more  rapidly  at  the  poles 
than  at  the  equator,  because  the  force  of  gravity  is  greater  at  the 
poles  than  at  the  equator. 

J-CJ',^00 

Questions  for  Review. 

By  what  is  the  weight  of  a body  caused?  State  the  law  of  uni- 
versal gravitation.  What  do  we  mean  when  we  say  that  one  thing  is 
inversely  proportional  to  another  ? 

If  the  mass  of  a body  be  doubled,  how  much  will  its  attractive 
force  be  increased  ? If  the  distance  between  two  bodies  be  doubled, 
how  much  will  their  attractive  force  be  decreased  ? 

In  what  direction  does  the  force  of  gravity  act  ? How  may  this 
direction  be  ascertained  ? 

Define  centre  of  gravity.  Why  may  the  centre  of  gravity  be  re- 
garded as  the  point  of  application  of  the  force  of  gravity  ? 

How  may  the  centre  of  gravity  he  ascertained  experimentally  ? 

Wliy  should  a body  supported  at  its  centre  of  gravity  be  in  equi- 
librium ? 

What  three  kinds  of  equilibrium  are  there?  "UTien  a body  is  sup- 
ported at  a point  around  which  it  is  free  to  move,  what  must  be  the 
relative  position  of  this  point  and  the  centre  of  gravity  for  the  body 
to  be  in  equilibrium  ? 

What  kind  of  equilibrium  will  exist  if  the  point  of  support  be  above 
the  centre  of  gravity?  At  the  centre  of  gravity?  Below  the  centre 
of  gravity? 

When  will  a body  resting  on  a flat  horizontal  surface  be  in  stable 
equilibrium  ? When  will  it  be  in  unstable  equilibrium  ? When  wOl 
it  be  in  neutral  equilibrium? 

Upon  what  does  the  degree  of  stability  of  a body  resting  on  a flat 
horizontal  surface  depend  ? 

State  the  laws  for  falling  bodies.  Wliy  should  the  velocity  of  a 
falling  body  be  independent  of  its  mass?  How  can  we  prove  that 
bodies  of  different  kinds  and  shapes  fall  with  the  same  velocity  ? 


QUESTIONS  FOR  REVIEW. 


81 


To  what  is  the  velocity  acquired  hy  a falling  hody  at  the  end  of  any 
given  time  proportional  ? Through  how  much  greater  distance  will  a 
hody  fall  during  the  second  second  of  its  descent  than  during  the  first 
second  ? To  what  are  the  total  spaces  fallen  through  proportional  ? 

Define  oscillation  of  a pendulum ; time  or  duration  of  an  oscillation ; 
and  amplitude  of  an  oscillation. 

In  the  same  pendulum,  when  the  amplitude  of  the  oscillation  is  not 
very  great,  does  the  pendulum  take  any  longer  time  to  move  through 
a long  arc  than  it  does  to  move  through  a short  one  ? 

How  does  the  length  of  the  pendulum  afiect  the  duration  of  its  oscil- 
lation ? 

How  can  we  determine,  hy  means  of  the  pendulum,  the  variations 
of  the  intensity  of  gravity  at  different  parts  of  the  earth  ? . 

F 


CHAPTER  VII. 

COHESION  AND  ADHESION,  AND  PROPER- 
TIES PECULIAR  TO  SOLIDS. 

84.  Force  of  Molecular  Attraction. — Tlie  force  of 
molecular  attraction  may  hold  together  molecules  of 
the  same  or  of  different  kinds  of  matter. 

Cohesion  is  the  name  given  to  the  force  of  molecu- 
lar attraction  when  it  holds  together  molecules  of  the 
same  kind  of  matter.  Adhesion  is  the  name  given  to 
it  when  it  holds  together  the  molecules  of  different 
kinds  of  matter. 

85.  Cohesion.  — The  force  of  cohesive  attraction 
binds  together  the  molecules  of  the  same  kind  of 
matter.  This  force  varies  greatly  in  different  sub- 
stances. In  some,  such  as  iron  and  steel,  the  cohesion 
is  very  great ; in  others,  such  as  soft  butter  or  puttv, 
the  cohesion  is  feeble.  It  is  the  cohesion  of  a solid 
that  causes  it  to  retain  its  shape. 

The  force  of  cohesive  attraction  appears  to  act  only 
at  very  small  distances.  If  we  once  overcome  the 
cohesion  between  the  particles  of  a piece  of  iron  or 
china,  we  cannot,  by  merely  pressing  the  broken  edges 
together,  cause  the  force  of  cohesion  to  again  bind 
them.  It  would  appear  as  if  we  could  not  bring  the 
molecules  sufficiently  near  one  another.  Two  fresh 

82 


K I 


COHESION  AND  ADHESION. 


83 


surfaces  of  lead,  however,  may  be  caused  to  cohere 
with  considerable  force,  by  being  merely  pressed  to- 
gether. Two  clean  plates  of  polished  glass,  such  as 
are  used  for  mirrors,  will  often  cohere  so  strongly, 
when  laid  one  on  the  other,  as  to  make  it  impossible 
to  separate  them  without  fracture. 

Experiment. — Cast  a cylinder  of  lead  about  one  inch  in  length  and 
a quarter  of  an  inch  in  diameter.  This  can  be  done  by  boring  a hole 
of  the  proper  size  in  a piece  of  hard,  dry  wood,  and  pouring  melted 
lead  into  the  hole.  Cut  the  cylinder  in  half 
by  resting  a sharp  knife  against  its  side  and 
striking  the  knife  a few  blows  with  a hammer. 

Attach  strings  or  wires  to  the  ends  of  the 
pieces,  as  shown  in  the  figure.  If  now  the 
freshly  cut  surfaces  be  firmly  pressed  to- 
gether, they  will  cohere  with  sufficient  force 
to  sustain  a heavy  weight  placed  below,  on  a 
pan  made  by  tying  strings  to  the  four  corners 
of  a square  piece  of  stout  pasteboard. 

Caution.  — Be  sure  that  the  mould  is  quite 
dry  before  pouring  in  the  lead.  Do  not  touch 
the  cut  surfaces  of  the  lead,  as  the  hands, 
even  when  clean,  will  tarnish  or  grease  the 
lead,  and  the  experiment  fail.  The  surfaces  Fig.  37.— The  Cohesion 
should  be  smooth.  If  the  cylinder  is  not  cut  Lead, 

smooth  by  the  knife,  it  may  be  filed  smooth,  and  then  made  bright 
and  clean  by  a knife.  Each  time  the  cylinders  are  used,  it  will  be 
necessary  to  reclean  the  surfaces. 

86.  Cohesion  of  Liquids. — ^Tbe  molecules  of  liq- 
uids move  over  one  another  so  easily  that  we  might 
suppose  that  they  possessed  no  cohesion.  They  do, 
however,  exert  a cohesive  attraction  for  one  another ; 
but  this,  of  course,  is  much  less  than  in  solids. 

Had  liquids  no  cohesion,  the  drops  of  dew  on  leaves 
would  be  pulled  by  gravity  into  thin  flat  layers,  in- 
stead of  having  their  almost  spherical  shape.  The 
force  of  cohesion  varies  in  different  liquids. 


84 


NATURA  L PHIL  OSO PH F. 


87.  Adhesion, — ^When  the  force  of  molecular  at- 
traction holds  together  the  molecules  of  different 
kinds  of  matter,  we  name  the  force  adhesive  attrac- 
tion, to  distinguish  it  from  cohesive  attraction.  Thus, 
the  hand  when  dipped  in  water  is  wet;  here  we  say 
the  water  adheres  to  the  hand,  because  the  attraction  is 
exerted  between  different  kinds  of  molecules,  namelv, 
between  those  of  the  hand  and  those  of  the  water. 
Chalk-marks  adhere  to  a blackboard,  but  the  molecules 
of  chalk  cohere  to  one  another. 

88.  Chemical  Attraction  or  Affinity. — The  force 
of  molecular  attraction  is  to  be  carefully  distinguished 
from  that  of  atomic  attraction  or  chemical  affinity.  The 
former,  as  we  have  seen,  may  attract  and  bind  together 
the  molecules  of  the  same  or  of  different  kinds  of 
matter.  The  latter  may  attract  and  bind  together  the 
atoms  of  the  same  or  of  different  kinds  of  matter.  It 
is  the  chemical  attraction  between  the  atoms  that  pro- 
duces the  molecules  of  elementary  and  of  compound 
substances. 

By  the  operation  of  molecular  attractions,  physical 
changes  are  produced  in  matter;  by  the  operation  of 
the  atomic  attractions,  chemical  changes  are  produced. 
Thus,  a piece  of  iron  exposed  to  moist  air  becomes  cov- 
ered with  rust.  This  rust  is  a compound  body,  and  is 
formed  by  the  atoms  of  the  iron  exerting  an  attraction 
for  the  atoms  of  oxygen  in  the  air.  The  body  formed 
by  the  combination,  or  union,  of  these  different  atoms, 
though  containing  both  iron  and  oxygen,  yet  possesses 
the  properties  of  neither ; its  constituents  have  under- 
gone a chemical  change. 

The  above  distinction  between  molecnlar  and  atomic  attractions  is 
not  in  all  cases  so  clear.  In  the  solution  of  certain  solids  by  liquids, 
there  appears  to  be  a combination  of  the  molecules  of  the  solid  and 


COEESION  AND  ADHESION. 


85 


liquid,  which  can  scarcely  he  regarded  as  either  purely  physical  or 
chemical.  A similar  combination  occurs  in  salts  which  possess  water 
of  crystallization.  These  combinations  might  be  regarded  as  partly 
chemical  combinations,  but  it  will  be  more  convenient  to  consider  all 
attractions  not  evidently  chemical,  as  varieties  of  adhesion. 

89.  Varieties  of  Adhesion. — The  force  of  adhesive 
attraction  manifests  itself  in  a variety  of  ways.  We 
will  consider  the  adhesion  which  occurs, 

1st.  Between  solids. 

2d.  Between  liquids. 

3d.  Between  solids  and  liquids. 

4th.  Between  solids  and  gases. 

5th.  Between  liquids  and  gases. 

90.  Adhesion  between  Solids.  — The  resistance 
which  friction  offers  to  motion  is  caused  not  only  hy 
the  irregularities  of  the  surfaces,  but  also  by  the  adhe- 
sion which  they  exert  for  one  another.  When  these 
surfaces  are  both  of  the  same  kind,  the  friction  is  in- 
creased by  their  cohesion. 

Cements  afford  examples  of  adhesion  between  dif- 
ferent kinds  of  solids.  Thus,  mortar  placed  between 
stones  or  bricks  adheres  to  them,  and  so  binds  them 
together.  Glue  adheres  to  the  pieces  of  wood  or  cloth 
between  which  it  is  placed.  Paste  or  gum  causes  paper 
to  adhere  to  walls.  Dried  paint  adheres  to  wood-work, 
and  ink-marks  to  paper. 

91.  Adhesion  between  Liquids. — i.  Mixture.  Oil 

and  water  will  not  mix,  because  there  is  but  little  ad- 
hesion between  their  molecules.  The  same  is  true  of 
mercury  and  water.  Milk  and  water,  however,  mix 
readily,  because  the  molecules  of  the  one  adhere  to 
those  of  the  other ; the  same  is  true  of  many  other 
liquids. 

8 


86 


NATURAL  PHILOSOPIIT. 


2.  Solution.  The  solution  of  one  liquid  by  another  may  also  be 
regarded  as  a species  of  adhesion.  Thus,  castor-oil  is  dissolved  by 
alcohol.  Many  cases  of  solution,  however,  are  caused  by  the  influence 
of  a partly  chemical  attraction.  Thus,  pure  concentrated  alcohol 
and  water  combine ; but  a weak  solution  of  alcohol  will  mix  with 
water  in  any  proportion. 

3.  Diffusion.  If  a vessel  filled  witli  liquid  fie  care- 
fully lowered  fieneatfi  tfie  surface  of  some  ligfiter  liquid 
with  wfiicfi  it  is  capafile  of  mixing,  tliough  no  agita- 
tion lias  occurred,  yet  after  a time,  which  differs  for  dif- 
ferent liquids,  the  two  will  be  found  to  be  thoroughly 
mingled,  as  much  of  the  heavier  liquid  being  now 
found  in  the  upper  portions  of  the  vessel  as  in  the 
lower.  Phenomena  of  this  kind  are  known  by  the 
general  name  of  diffusion,  and  are  caused  by  the  at- 
tractions which  exist  between  the  unlike  molecules. 

92.  Adhesion  between  Solids  and  Liquids. — i. 
Wetting.  If  we  plunge  the  hand  into  water,  it  be- 
comes wet,  because  the  water  adheres  to  it ; but  if 
plunged  into  mercury,  it  is  not  wet,  because  there  is 
but  little  adhesion  between  the  hand  and  the  mercury. 
Water-proof  stuffs  contain  substances  that  are  not 
readily  wet  by  water.  When  rain  falls  on  such  fabrics, 
it  either  runs  off  of  itself,  or  can  be  easily  shaken  off". 

2.  Solution.  The  solution  of  a solid  hry  a liquid  is 
another  example  of  adhesion  between  a solid  and  a 
liquid. 

When  a lump  of  sugar  is  thrown  into  a glass  of 
water,  the  sugar  gradually  disappears  and  becomes 
mixed  throughout  the  liquid,  which  has  now  a sweet- 
ish taste.  We  call  this  change  solution;  by  solution 
a solid  becomes  changed  into  a liquid. 

The  solvent  powers  of  water  are  much  greater  than  those  of  any 
other  common  liquid.  As  a rule,  the  solvent  power  of  any  substance 


COHESION  AND  ADHESION 


87 


increases  with  the  temperature ; thus,  hot  water  will  dissolve  more 
sugar  than  cold  water.  This  is  not.  however,  always  the  case  ; cold 
water  will  dissolve  more  lime  than  hot  water.  Some  cases  of  solution 
are  caused  by  the  influence  of  a species  of  chemical  attraction,  such, 
for  example,  as  the  solution  of  concentrated  lye  by  water. 


Fig.  38 The  Liquid  Wets 
the  Tube, 


93.  Capillarity. — If  a tube,  J.,  of  large  diameter  be 
dipped  into  a liquid  wbicb  wets  it,  tlie  liquid  will  stand 
as  high  on  the  outside  of  the 
tube  as  on  the  inside;  but  if 
the  tube  be  of  small  diameter, 
as  at  the  liquid  will  rise  so 
that  it  will  be  higher  inside  the 
tube  than  outside  it,  as  is  shown 
in  Fig.  38.  If  the  tube  is  still 
smaller,  as  at  O',  the  liquid  will 
rise  higher  than  before,  aud  the 
difference  between  the  inside  and  outside  levels  will 
be  greater. 

When  the  liquid  does  not  wet  the  walls  of  the  tube, 
if  the  tube  be  of  large  diam- 
eter, as  shown  at  Fig.  39, 
the  level  inside  the  tube  is 
the  same  as  that  outside  it; 
but  if  the  diameter  be  small, 
as  at  F",  the  liquid  will  be 
depressed  within  the  tube, 
so  that  the  level  of  the  liq- 
uid inside  the  tube  will  be 
below  that  on  the  outside.  In  a tube  of  still  smaller 
diameter,  as  at  (7,  the  difference  of  level  is  greater. 
These  phenomena  are  known  by  the  name  of  ca^jillar- 
ity.  A capillarij  tube  is  one  whose  diameter  is  small, 
or  hair-like. 


Fig.  39,- 


-The  Liquid  does  not  Wet 
the  Tube. 


88 


NATURAL  PHILOSOPHY. 


By  capillarity  is  meant  the  elevation  or  the  depression 
of  liquids  in  tubes  of  small  diameter. 

The  principal  phenomena  of  capillarity  may  be 
briefly  stated  as  follows,  viz. ; 

1st.  When  a capillary  tube  is  dipped  into  a liquid, 
the  liquid  will  rise  in  the  tube  if  it  wets  the  tube,  but 
will  be  depressed,  if  it  does  not  wet  the  tube. 

2d.  The  amount  of  the  elevation  or  depression  in- 
creases as  the  diameter  of  the  tube  becomes  smaller. 

3d.  The  amount  of  elevation  or  depression  varies 
with  the  kind  of  liquid  and  the  material  of  the  tube. 

94.  The  Cause  of  Capillary  Phenomena. — Capil- 
lary phenomena  are  caused  by  the  difference  between 
the  cohesion  of  the  liquid  molecules  for  one  another 
and  their  adhesion  to  the  walls  of  the  capillary  tube. 


A liquid  rises  in  a capillary  tube  ■which  it  wets,  because  the  adhesion 
between  the  liquid  and  the  walls  of  the  tube  draws  the  liquid  towards 
the  walls  of  the  tube.  A liquid  is  depressed  in  a capillary  tube  which 
it  does  not  wet,  because  the  cohesion  of  the  liquid  draws  the  liquid 
away  from  the  walls  of  the  tube.  If  the  liquid  wets  the  tube,  then 
the  adhesion  between  it  and  the  tube,  or  the  force  which  pulls  the  liq- 
uid towards  the  walls,  is  greater  than  the  force  of  cohesion  which 
tends  to  keep  the  particles  of  the  liquid  together.  If  the  liquid  does 
not  wet  the  tube,  then  the  force  of  cohesion  is  greater  than  that  of 
adhesion. 


95.  Familiar  Examples  of  Capillarity. — The  phe- 
nomena of  capillarity  occur  in  loose,  porous  substanees 
whenever  the  spaces,  or  sensible  pores,  between  the 
different  parts  of  the  substance  are  of  capillary  dimen- 
sions. Thus,  the  oil  rises  in  the  wick  of  a lamp  through 
the  capillarity  of  the  spaees  between  the  strands.  A 
lump  of  sugar  placed  Avith  only  its  lower  end  in  milk 
or  Avater  is  soon  Avet  throughout ; here  the  elevation 


COHESION  AND  ADHESION. 


89 


of  the  liquid  is  clue  to  capillarity.  A towel  hung  so 
that  only  its  lower  part  clips  into  water  is  soon  wet  for 
a considerable  distance  above  the  level  of  the  water, 
from  the  same  cause. 

96.  Osmose  is  the  unequal  mixing  of  two  different 
liquids  through  the  pores  of  a membranous  substance 
which  separates  them. 

If  two  liquids  that  are  capable  of  mixing,  be  placed  in  compart- 
ments of  the  same  vessel,  and  separated  from  each  other  by  only  a 
thin  wall  of  bladder  or  other  membrane,  through  the  pores  of  which 
the  liquids  can  slowly  pass,  they  will  not  remain  separate,  but  will 
mix  with  each  other.  This  mixing,  however,  is  different  from  that 
produced  by  mere  diffusion,  since  more  liquid  passes  in  one  direction 
than  in  the  other.  Suppose,  for  example,  sugar  and  water  were  placed 
in  one  compartment,  and  pure  water  in  the  other ; then  it  would  be 
found  that  more  pure  water  would  pass  into  the  compartment  contain- 
ing the  sugar  and  water,  than  the  sugar  and  water  would  pass  into  the 
other  compartment,  so  that,  after  standing  for  several  hours,  the  level 
of  the  liquid  would  he  higher  in  the  compartment  containing  the 
sugar  and  water  than  in  the  other. 

The  cause  of  osmose  is  not  thoroughly  understood.  It  is  probable, 
however,  that  these  phenomena  are  due  mainly  to  the  adhesion  which 
exists  both  between  the  molecules  of  the  two  liquids  and  between 
them  and  the  walls  of  the  membrane. 

97.  Adhesion  between  Solids  and  Gases.  — Tbe 

adbesion  existing  between  solids  and  gases  is  seen  in 
tbe  absorption  of  gases  by  solids.  Charcoal,  for  exam- 
ple, has  a wonderful  power  of  absorbing  various  gases 
and  condensing  them  within  its  pores.  Freshly -burned 
charcoal  can  absorb  nearly  a hundred  times  its  bulk  of 
some  gases.  When  thrown  on  decaying  animal  or 
vegetable  substances,  it  removes  most  of  their  bad 
odors  by  absorbing  the  disagreeable  gases  as  fast  as 
they  are  given  off.  The  smell  of  tobacco  clings  for 
some  time  to  the  clothes  of  one  who  has  been  smok- 
8* 


90 


NATURAL  PHILOSOPHY. 


f 


ing.  This  is  due  to  the  adhesion  between  the  smoke 
and  the  clothes. 

98.  Adhesion  between  Liquids  and  Gases. — Most 
liquids  have  the  power  of  absorbing  various  gases. 
W ater  possesses  this  property  in  a remarkable  degree. 
All  water  which  has  been  exposed  for  some  time  to 
the  air  will  be  found  to  contain  a considerable  amount 
of  air  in  solution.  If  a tumbler  of  clear  Avater  stand 
for  some  time,  minute  bubbles  of  gas  Avill  be  seen  on 
the  inside  of  the  glass.  These  bubbles  come  from  the 
air  which  was  contained  in  solution  in  the  Avater. 

99.  Properties  Peculiar  to  Solids. — The  solid  con- 
dition of  matter  is  characterized  by  certain  properties 
peculiar  to  it.  The  most  important  properties  pecu- 
liar to  solids,  are  malleability.,  ductility,  hardness,  brit- 
tleness, tenacity,  solid  elasticity,  and  crystalline  form. 

100.  Malleability  is  the  property  certain  solids 
possess  of  being  wrought  into  different  shapes  under 
the  hammer  or  roller.  Most  of  the  metals  are  malle- 
able to  a considerable  extent,  and  on  this  property 
much  of  their  value  depends.  Gold  is  one  of  the 
most  malleable  metals  knoA\m.  It  can  be  beaten  into 
leaves  so  thin,  that  it  takes  about  300,000  such  leaA’cs 
to  make  a pile  one  inch  in  thickness.  The  malleabil- 
ity of  metals  under  the  hammer  is  someAvhat  different 
than  under  the  roller.  Gold,  lead,  silver,  tin,  and  cop- 
per are  very  malleable. 

101.  Ductility  is  the  property  certain  solids  pos- 
sess of  being  draAA'n  out  into  Avire.  This  property, 
Avhich  is  possessed  most  generally  by  the  metals,  is 
nearly  the  smne  as  malleability,  since  in  both,  the 
molecules  of  the  body,  AA’hen  subjected  to  pressure  or 
strain,  are  caused  to  floAV  or  moAm  OA'er  one  another 


COHESION  AND  ADHESION. 


91 


without  fracture.  The  different  metals,  however,  are 
not  malleable  and  ductile  to  the  same  extent. 

Platinum,  silver,  iron,  and  copper  are  very  ductile. 

102.  Hardness  is  that  property  in  virtue  of  which 
certain  substances  resist  being  scratched  or  worn  by 
others.  We  can  tell  which  of  two  substances  is  the 
harder,  by  rubbing  them  together ; that  Avhich  is  the 
harder  will  scratch  the  other.  The  terms  hard  and 
soft  are  relative,  since  a body  which  is  hard,  when 
compared  with  one  substance,  may  be  soft  when  com- 
pared with  another ; thus,  glass  will  scratch  marble, 
therefore  glass  is  hard  when  compared  with  marble, 
but  the  diamond  will  scratch  glass,  so  that  glass  is  soft 
when  compared  with  the  diamond. 

Hardening,  Annealing,  Tempering. — Certain  metals  possess  the  re- 
markable property  of  having  their  hardness  changed  by  heat.  If 
they  are  heated  to  about  redness,  and  then  suddenly  cooled  by  being 
plunged  into  cold  water,  they  become  hard  and  brittle.  Steel  pos- 
sesses this  property  in  a remarkable  degree.  The  process  is  called 
hardening. 

When  steel  is  highly  heated  and  allowed  to  cool  slowly,  it  becomes 
soft,  ductile,  and  malleable.  This  process  is  sometimes  called  softening 
or  annealing. 

If  a bar  of  hardened  steel  is  struck  a sharp  blow  with  a hammer,  it 
is  apt  to  be  fractured ; but  if  softened,  it  can  be  wrought  into  any 
desired  shape,  such,  for  example,  as  a knife-blade,  an  axe,  or  a hatchet, 
and  these  articles  can  afterwards  be  hardened  by  high  heating  and 
subsequent  rapid  cooling.  As  a rule,  the  hardness  so  obtained  is  so 
great  that  the  articles  would  be  too  brittle  for  use;  in  this  case,  a 
portion  of  the  hardness  may  be  removed  by  heating  to  a lower  tem- 
perature, and  then  allowing  them  to  cool.  This  process  is  called  draw- 
ing the  temper. 

103.  Brittleness. — A substaace  is  brittle  when  it  is 
readily  broken  into  pieces.  This  property  is  almost  the 
opposite  to  that  of  malleability.  Brittle  substances  are 
generally  hard. 


92 


NATURAL  PHILOSOPHY. 


104.  Tenacity. — By  the  tenacity  of  a substance  is 
meant  its  power  to  resist  being  pulled  apart  by  a force 
acting  in  the  direction  of  its  length.  The  tenacity  of 
a body  is  due  to  the  cohesion  of  its  molecules,  and, 
as  cohesive  attraction  varies  greatly  in  different  sub- 
stances, the  tenacity  must  always  vary. 

As  the  force  tending  to  pull  the  molecules  apart  is 
applied  in  the  direction  of  the  length  of  the  body,  the 
tenacity  must  increase  with  the  area  of  transverse  sec- 
tion, that  is,  a section  made  at  right  angles  to  the  length ; 
for,  if  in  the  two  bars,  A and  5,  Fig.  40,  of  the  same 
material,  A has  a smaller  area  of  transverse  section, 
abed,  than  efgh  of  B,  then  the  total  tenacity  of  A 
must  be  less  than  that  of  B,  since  there  are  fewer  mole- 
cules in  any  cross  section,  as  abed,  tending  to  hold 
the  bar  together  than 
there  are  in  any  cross 


d h 

Fig.  40. — Influence  of  Sectional  Area  on  Tenacity. 

section  efgh.  If,  then,  the  area  efgh  be  twice  as 
great  as  abed,  the  bar,  B,  would  require  twice  as 
great  a force  to  pull  it  apart  as  the  bar  A,  and  so  also 
with  any  other  proportion. 

The  tenacity  of  a bar  or  beam  is  independent  of  its 
length.  The  increase  in  the  length  does  not  affect  the 
number  of  molecules  in  any  area  of  cross  section,  and, 
of  course,  the  bar  would  break  at  its  weakest  part. 
Since,  however,  the  weight  of  a beam,  supported  at 
one  end,  tends  to  break  the  beam,  we  can  see  that  the 
brealcing  weight  is  less  with  an  increased  length. 

In  the  following  table  is  given  the  tenacity  of  a few  important 
substances  in  pounds  per  square  inch  of  area  of  cross  section. 


I 


It 


COHESION  AND  ADHESION. 


93 


Steel  . 120,000  to  160,000 

Iron  . . 50,000  to  120,000 
Silver  ....  41,000 

Copper  ....  37,000 


Gold 

Zinc 


65,000 

2,800 


Oak  wood  . . . 26,000 
Pine  wood  . . . 15,000 


105.  Limits  of  the  Size  of  Structures. — The  size  of  any  structure 
cannot  exceed  certain  limits,  which  vary  with  the  strength  of  material 
employed  and  with  its  weight  per  unit  of  volume : for,  since  all  ma- 
terials have  to  sustain  their  own  weight,  if  this  should  exceed  their 
tenacity,  the  structure  would  fall.  We  have  seen  that  the  tenacity 
of  a body  increases  as  the  area  of  its  cross  section.  If,  then,  the 
dimensions  of  a beam  be  doubled,  that  is,  if  it  be  made  twice  as  long, 
twice  as  deep,  and  twice  as  broad,  the  area  of  its  cross  section  will  be 
four  times  as  great,  and  it  will  be  able  to  sustain  four  times  as  great  a 
weight  acting  in  the  direction  of  its  length.  But  at  the  same  time,  by 
doubling  its  dimensions,  we  have  increased  its  volume  eight  times ; 
hence  its  weight  has  also  been  increased  eight  times.  If  we  increase 
the  dimensions  of  the  beam  three  times,  its  tenacity  is  3 X 3 = 9 times 
greater,  and  its  weight,  3X3  X3  = 27  times  greater ; if  increased  four 
times,  its  tenacity  is  4 X 4 = 16  times,  and  its  weight  4 X 4 X 4 = 64 
times.  It  is  apparent,  then,  that  any  bar  would  soon  become  large 
enough  to  fall  apart  by  its  own  weight. 


106.  Elasticity.  — By  the  elasticity  of  a body  we 
mean  the  property  it  possesses  of  regaining  its  orig- 
inal shape  after  being  compressed,  stretched,  bent,  or 
twisted. 

All  kinds  of  matter,  whether  solid,  liquid,  or  gas- 
eous, will,  when  compressed,  tend  to  some  extent  to 
regain  their  original  bulk,  or,  in  other  words,  possess 
the  property  of  elasticity. 

Elasticity  developed  in  a hody  hy  compression  is 
therefore  a general  property  of  matter. 

Solids  are  the  only  form  of  matter  which  can  be 
stretched,  bent,  or  twisted,  and  by  any  of  these 
changes  elasticity  is  developed. 

Elasticity  developed  in  a hody  hy  stretching,  bending, 
or  twisting,  is  therefore  a property  peculiar  to  the  solid 
condition  of  matter. 


94 


NATURAL  PHILOSOPHY. 


Elasticity  developed  hy  stretching.!  or  tension,  is  seen 
in  the  return  to  its  former  length  of  a piece  of  caout- 
chouc, or  India-rubber,  or  a wire  after  it  has  been 
stretched  by  a weight.  Elasticity  developed  hy  bend- 
ing, or  flexure,  is  seen  in  the  recoil  of  a bow  which  has 
been  bent  and  then  released.  Elasticity  developed  hy 
twisting,  or  torsion,  is  seen  in  the  untwisting  of  a string 
or  wire  which  has  been  twisted. 

107.  The  Measure  of  Elasticity. — The  degree  of 
elasticity  of  a body  is  measured  by  the  force  with  wdiich 
it  tends  to  regain  its  original  shape,  when  that  shape 
has  been  changed  by  compression,  stretching,  bending, 
or  twisting.  A body  which  in  resuming  its  original 
form  gives  out  a force  equal  to  that  by  which  its 
form  has  been  changed,  is  said  to  be  perfectly  elastic. 
Liquids  and  gases,  as  a rule,  when  compressed,  resume 
their  original  volume  on  the  removal  of  the  pressure, 
and  are  therefore  elastic.  Within  certain  limits  of 
compression,  etc.,  nearly  every  solid  body  will  resume 
its  original  shape  on  the  relief  of  compression,  bend- 
ing, stretching,  or  twisting,  and  is  therefore  elastic ; 
but  with  many  solids,  the  limits  of  elasticity,  or  the 
limit  beyond  which  aiiy  further  compression,  bending, 
stretching,  or  twisting  would  produce  a permanent 
change  of  form,  are  so  small  that  they  may  be  consid- 
ered as  inelastic. 

108.  Crystalline  Form. — We  recognize  an  animal 
or  a plant  by  a certain  form  peculiar  to  it.  In  the 
same  way  many  lifeless  substances  occur  in  forms, 
called  crystals,  peculiar  to  them.  Although  in  most 
solids  these  crystals  are  too  small  to  be  seen,  yet  they 
nearly  always  exist.  For  example,  ice  is  composed  of 
crystals  of  beautiful  shapes. 


COHESION  AND  ADHESION. 


95 


Crystals  are  more  or  less  regular  in  shape,  and 
although  of  many  different  forms,  yet  are  all  modifica- 
tions of  a few  simple  forms. 

Experiment. — Place  a quarter  of  a pound  of  common  alum  in  as 
much  hot  water  as  will  completely  dissolve  it.  Strain  the  solution 
through  a piece  of  muslin,  and  pour  the  clear  liquid  into  a cup  or 
howl,  iu  which  has  been  placed  a piece  of  rough  stone  wrapped  with 
colored  yarn.  Set  the  liquid  aside  in  a quiet  place  over  night,  and 
in  the  morning  beautiful  shining  crystals  will  be  found  covering  the 
stone.  By  slipping  a thin  knife  under  the  stone  it  may  be  separated 
from  the  bottom  of  the  cup  or  bowl. 

Caution. — Do  not  move  or  shake  the  cup  after  once  putting  it  away. 
If  it  is  wished  to  obtain  large  crystals,  use  more  water  to  dissolve  the 
alum,  and  let  it  stand  longer  to  crystallize. 

The  force  of  crystallization  which  arranges  the  molecules  of  solids  in 
the  form  of  regular  crystals,  is  only  a variety  of  the  force  of  cohesive 
attraction. 




Syllabus. 

The  force  of  molecular  attraction  when  acting  on  the  molecules  of 
the  same  kind  of  matter,  is  called  cohesive  attraction,  and  when  acting 
on  the  molecules  of  different  kinds  of  matter,  adhesive  attraction. 

The  force  of  cohesive  attraction  is  manifested  principally  by  solids  ; 
it  is  not  absent,  however,  in  liquids. 

The  force  of  molecular  attraction  binds  together  molecules  of  either 
the  same  or  of  different  kinds  of  matter.  The  force  of  atomic  attrac- 
tion, or  chemical  affinity,  binds  together  the  atoms  of  either  the  same 
or  of  different  kinds  of  matter. 

The  force  of  adhesion  may  exist  between  all  three  forms  of  matter, 
viz.,  solids,  liquids,  and  gases. 

The  friction  of  two  surfaces  is  caused  partly  by  their  adhesion.  Such 
substances  as  glue,  paste,  paint,  and  ink  owe  their  efficiency  to  their 
power  of  adhering  to  the  surfaces  on  which  they  are  put. 

Water  and  vinegar,  or  water  and  milk,  mix  when  put  together,  be- 
cause they  adhere  to  one  another ; oil  and  water,  or  water  and  mer- 
cury, do  not  mix,  because  they  do  not  adhere. 

Diffusion  is  that  property  in  virtue  of  which  two  different  gases,  or 
two  different  liquids,  will  mix  with  each  other,  even  when  the  lighter 
liquid  or  the  lighter  gas  is  placed  above  the  other. 


96 


NATURAL  PIIILOSOPnY. 


It  is  by  the  action  of  diffusion  that  the  different  gases  which  com- 
pose our  atmosphere  are  kept  from  settling  in  layers,  according  to 
their  difference  of  density. 

A solid  is  dissolved  by  a liquid,  when  the  adhesion  between  the  two 
is  sufficient  to  overcome  the  cohesion  between  the  molecules  of  the 
solid. 

By  capillarity  we  mean  the  elevation  or  the  depression  of  liquids  in 
tubes  of  small  internal  diameter.  When  the  liquid  wets  the  tube,  it  is 
elevated  in  it ; when  it  does  not  wet  it,  the  liquid  is  depressed. 

When  the  liquid  wets  the  tube,  the  adhesion  between  the  lube  and 
the  liquid  is  greater  than  the  cohesion  between  the  molecules  of  the 
liquid,  and  the  liquid  is  therefore  drawn  up  the  tube.  When,  how- 
ever, the  cohesion  of  the  liquid  is  greater  than  its  adhesion  to  the 
tube,  the  liquid  is  drawn  down  the  tube. 

Osmose  is  the  unequal  mixing  of  two  liquids  through  the  pores  of  a 
membrane  or  wall  separating  them. 

Many  solids  possess  the  power  of  absorbing  gases  and  condensing 
them  ; this  power  is  due  to  the  adhesion  between  the  solid  and  the  gas. 
Most  liquids  possess  the  same  property. 

The  most  important  of  the  properties  peculiar  to  solids  are  mallea- 
bility, ductility,  hardness,  brittleness,  tenacity,  solid  elasticity,  and 
crystalline  form. 

Many  solids,  when  subjected  to  great  pressure,  exhibit  to  some  degree 
the  phenomena  of  flow  ; they  are  malleable  or  ductile,  that  is,  can  be 
beaten  out  in  thin  sheets,  and  pulled  out  into  thin  wires  without  frac- 
ture. These  properties  are  called  malleability  and  ductility. 

Solids  differ  in  their  hardness,  or  their  ability  to  resist  being  worn 
or  scratched.  Certain  metals  when  highly  heated  and  then  suddenly 
cooled  become  hard  and  brittle,  but  w'hen  heated  and  then  slowly 
cooled  become  soft  and  ductile.  Brittle  substances  are  easily  broken 
into  small  pieces  when  struck. 

The  tenacity  of  a bar  or  rod  is  its  power  to  resist  being  pulled  apart 
by  a force  applied  in  the  direction  of  its  length.  The  tenacity  is  greater 
in  some  substances  than  in  others.  In  the  same  substance  the  tenacity 
is  proportional  to  the  area  of  cross  section. 

By  the  elasticity  of  a body  we  mean  its  tendency  to  resume  its  orig- 
inal bulk  on  the  removal  of  any  force  which  has  acted  to  change  its 
shape.  Elasticity  may  be  developed  in  all  bodies  by  compression, 
and  in  solids  by  stretching,  bending,  or  twisting. 

Most  solids  have  a peculiar  shape,  or  crystalline  form,  which  is  char- 
acteristic of  them.  This  shape  is  caused  by  the  force  of  molecular 
attraction  acting  in  certain  directions  on  the  molecules. 


QUESTIONS  FOR  REVIEW. 


97 


Questions  for  Review. 

Define  cohesion,  adhesion.  What  is  the  difiference  between  them? 
Describe  an  experiment  by  which  the  cohesion  of  two  pieces  of  lead 
may  be  shown. 

What  facts  can  you  mention  to  prove  that  liquids  possess  cohesion  ? 

What  is  the  difference  between  molecular  attraction  and  chemical 
affinity  ? Name  five  varieties  of  adhesion.  Explain  some  of  the  phe- 
nomena produced  by  the  adhesion  of  solids. 

What  is  meant  by  diffusion?  What  two  kinds  of  diffusion  are 
there  ? Of  what  use  is  diffusion  in  our  atmosphere  ? 

By  what  is  the  solution  of  a body  caused? 

What  do  you  understand  by  capillarity?  Why  should  a liquid  be 
elevated  in  a capillary  tube  when  the  liquid  wets  the  tube  ? Why 
should  it  be  depressed  when  the  liquid  does  not  wet  the  tube  ? Name 
some  phenomena  produced  by  capillarity. 

Wliat  is  osmose  ? When  does  it  occur  ? Define  absorption.  To 
what  do  solids  owe  their  power  of  absorption  ? Do  liquids  possess 
the  power  of  absorbing  gases  ? 

Define  malleability,  ductility.  How  do  these  properties  prove  that 
solids  possess  to  some  extent  the  power  of  flowing  ? 

What  do  you  understand  by  the  hardness  of  a substance  ? Prove 
that  hard  and  soft  are  relative  terms.  Describe  the  process  of  harden- 
ing ; of  annealing  ; of  drawing  the  temper  Define  brittleness. 

What  do  you  understand  by  the  tenacity  of  a substance  ? What 
effect  is  produced  on  the  tenacity  of  a beam  by  increasing  the  area 
of  its  cross  section  ? By  increasing  its  length  ? 

What  natural  limit  exists  to  the  size  of  structures  ? Explain  your 
answer  in  full. 

What  is  meant  by  elasticity?  How  may  it  be  developed  in  all 
kinds  of  matter  ? How  may  it  be  developed  in  solids  ? 

How  is  the  elasticity  of  a body  measured  ? What  is  meant  by  the 
limits  of  elasticity  ? 

Define  crystalline  form.  Describe  an  experiment  by  which  crystals 
of  alum  can  be  formed. 

9 G 


Part  IL 

Fluids. 

CHAPTER  I. 

HYDROSTATICS. 

109.  Hydrodynamics  and  its  Divisions. — Hydro- 
dynamics is  that  branch  of  Natural  Philosophj"  which 
treats  of  the  conditions  of  rest  and  motion  in  fluid 
bodies.  It  includes  Hydrostatics,  Avhich  treats  of  liq- 
uids at  rest  ; Hydraulics,  which  treats  of  liquids  in 
motion,  and  Pneumatics,  which  treats  of  gases  either 
at  rest  or  in  motion. 

110.  Compressibility  of  Liquids. — Liquids  are  but 

slightly  compressible.  Thus,  Avater,  when  subjected  to 
a pressure  of  fifteen  pounds  to  the  square  inch,  is  com- 
pressed but  the  of  its  volume.  Many  other  liq- 

uids are  even  less  compressible.  We  may  therefore 
regard  liquids  as  practically  incompressible. 

Liquids,  hoAveA’er,  are,  as  a rule,  more  compressible  than  solids. 
When  solids  are  compressed,  they  are  not  generally  confined  at  the 
sides,  and  the  particles  spread.  They  appear,  therefore,  to  he  more 
compressible  than  liquids.  If  -we  should  confine  solids  in  strong  A'es- 
sels,  so  as  to  prevent  them  from  spreading  laterally,  as  we  must  do 
with  liquids,  Ave  Avould  find  solids  to  he  less  compressible  than  liquids. 

98 


HYDROSTATICS. 


99 


111.  Transmission  of  Pressure.  — Law.  Liquids 
transmit  in  all  directions.^  and  vdtliout  sensible  loss 
of  intensity.!  the  pressure  exerted  on  any  part  of  their 
7nass. 

Liquids  transmit  pressure  equally  in  all  directions 
as  a necessary  result  of  tlie  very  great  freedom  with 
Avhich  their  molecules  move  in  any  direction  over  one 
another. 

The  pressure  due  to  the  weight  of  a solid  is  exerted  only  in  one 
direction,  viz.,  vertically  downwards.  A solid,  therefore,  may  be  in 
equilibrium  when  supported  only  at  its  base.  The  pressure  due  to  the 
weight  of  a liquid  is  exerted  in  all  directions.  A liquid,  therefore,  to  be 
in  equilibrium,  must  be  supported  at  the  sides  as  well  as  at  the  base. 

112.  Liquid  Pressure  as  a Mechanical  Power. — 

Since  pressure  is  transmitted  through  liquids  as  well 
in  one  direction  as  in  another,  and  as  nothing  is  lost 
during  the  transmission,  it  follows  that  the  total  press- 
ure sustamed  by  any  surface  is  proportional  to  its  area. 
This  fact  furnishes  us  with  an  additional  mechanical 
power. 

If,  for  example,  two  vessels,  A and  B,  Fig.  41,  filled 

with  water,  he  furnished  with 

pistons,  C and  1),  that  is,  with 

parts  arranged  so  as  to  move  ^ 

freely  up  or  down  the  vessels 

without  allowing  the  water  to  ^ 

pass  them,  and  these  vessels 

he  connected  by  a tube,  L Tig.  41,  - Liquid  Pressure  as  a 
. y , . Meohauical  Power, 

we  have  a simple  machine  by 

which  we  can  modify  the  effects  of  a force  to  any 
desired  extent.  Suppose  the  areas  of  the  two  pistons, 
C and  ii,  he  respectively  1 and  100  square  inches.  If, 
then,  the  piston,  C',  he  pushed  down  with  a force  of  1 
lb.,  the  piston,  D,  will  be  raised  with  a force  of  100 


100 


NATURAL  PHILOSOPHY. 


lbs. ; for,  since  a pressure  of  1 lb.  is  exerted  at  C on 
the  area  of  1 sq.  in.,  it  must  exert  a pressure  of  1 lb. 
on  every  square  incb  of  surface  in  the  two  ve.ssels. 
But  the  piston,  /),  is  the  only  other  part  of  the  vessel 
that  can  move,  and,  as  it  contains  100  square  inches,  it 
must  be  pushed  upwards  with  a force  of  100  lbs. 

As  with  any  other  machine  in  which  the  intensity  of  the  power  is 
increased,  the  distance  throngli  which  the  power  moves  is  as  much 
greater  than  that  through  which  the  weight  moves  as  the  power  is 
less  than  the  weight ; if,  for  example,  the  piston.  C.  he  moved  by  the 
power  through  1 foot,  the  piston,  D,  will  only  raise  the  weight  through 
the  of  a foot. 


113.  The  Hydrostatic  Press.  — The  macbine  just 
described  forms  wbat  is  known  as  tbe  hydrostatic  press. 
A lever,  P,  Fig.  42,  is  attached  to  the  piston.  A,  and  by 

its  movement  oil  or  water 
is  pumped  iuto  the  larger 
vessel  in  which  the  pis- 
ton, .A,  moves.  The  jires- 
sure  thus  exerted  causes 
the  piston,  A,  to  rise.  Sub- 
stances to  be  compressed, 
such,  for  example,  as  hay 
or  cotton,  are  placed  be- 
tween a platform,  (?,  at- 
tached to  the  piston,  B, 
and  a strong  frame,  Z),  attached  to  the  press.  This 
press  is  used  to  compress  various  substances,  to  extract 
oil  from  seed,  to  raise  heavy  weights,  and  for  various 
other  purposes. 


Fig.  42.— The  Hydrostatic  Press. 


114.  Pressure  Caused  by  the  Weight  of  a Liquid. 

— Each  molecule  of  liquid  has  to  bear  the  weight  of 
all  the  molecules  directly  above  it.  The  greater  the 
depth  of  the  liquid,  the  greater  the  number  of  these 


HYDROSTA  TICS. 


101 


molecules.  The  pressure.^  therefore.,  exerted  hy  any  liq- 
uid increases  with  the  depth  of  the  liquid.  But  diftereut 
liquids  vary  iu  tlieir  densities,  that  is,  a cubic  inch  of 
some  liquids  weighs  more  than  a cubic  inch  of  others. 
The  pressure,  therefore,  exerted  hy  any  liquid  increases 
loith  its  density. 

A vessel  filled  with  mercury  or  molasses,  would 
have  a greater  pressure  on  its  bottom  and  sides  than  if 
it  were  filled  wifh  water,  because  mercury  or  molasses 
is  denser  than  water. 

As  liquids  exert  pressure  equally  well  in  all  direc- 
tions, the  downward,  upward,  and  lateral  pressure  at 
any  point  must  be  equal  to  one  another. 

115.  The  Downward  Pressure,  or  the  Pressure  on 
the  Base. — If  a vessel  with  vertical  Avails  be  filled  with 
liquid,  the  pressure  oai  its  base  Avill  be  equal  to  the 
weight  of  the  liquid  in  the  vessel ; but  if  the  walls  be 
inclined,  the  pressure  on  the  base  may  be  either  greater 
or  less  than  the  weight  of  the  Avater  in  the  vessel. 

In  the  vessel.  A,  a b a'  v a"  b" 

Fig.  43,  whose  walls 
are  vertical,  the  pres- 
sure on  the  base  is 
equal  to  the  Aveight 
of  Avater  in  the  vessel,  c d c'  d'  c"  d" 

In  B,  the  pressure  on  ^3.— The  Pressure  on  the  Base. 

the  base  is  greater  than  the  weight  of  the  Avater  in  B, 
and  in  C the  pressure  on  the  base  is  less  than  the 
Aveight  of  the  water  in  C. 

It  might  be  supposed  that  the  pressure  on  the  base 
of  any  vessel  Avould  ahvays  be  equal  to  the  Aveight  of 
the  liquid  in  the  vessel ; but  in  the  vessel,  B,  the  par- 
ticles at  the  surface  not  only  transmit  their  pressure 
9* 


102 


NATURAL  PHILOSOPHY. 


to  that  part  of  the  base  directly  under  them,  but  also 
to  all  parts  of  the  base  c'  cZ';  the  total  pressure  on  the 
base  is,  therefore,  the  same  as  if  there  were  the  same 
depth  of  liquid  over  all  parts  of  the  base  as  there  is 
over  the  middle  parts ; that  is,  the  total  pressure  is 
equal  to  the  weight  of  the  column  of  water,  a'  h'  c' d' . 
In  the  vessel,  C,  only  the  particles  of  the  liquid  imme- 
diately above  the  base  exert  a pressure  on  the  base ; 
the  inclined  walls  receive  the  pressure  of  the  remain- 
ing particles. 

If  the  three  vessels,  A,  B,  and  have  equal  bases, 
and  are  fdled  with  water  to  the  same  height,  the  press- 
ures on  the  bases  of  all  will  be  the  same. 

116.  Rule  for  Calculating  the  Pressure  on  the 
Base.  — The  pressure  on  the  base  of  a vessel  contain- 
ing liquid,  is  equal  to  the  weight  of  a column  of  liquid 
whose  base  is  that  of  the  vessel,  and  whose  height  is 
equal  to  the  vertical  distance  from  the  middle  of  the 
base  to  the  surface  of  the  liquid. 

The  weight  of  a cubic  foot  of  water  is  about  equal 
to  62.3  lbs.  avoirdupois ; the  weight  of  a cubic  inch  is 
.036  lbs. 

If,  therefore,  a vessel,  whose  base  has  an  area  of  2 sq. 
feet,  be  filled  with  water  to  the  depth  of  3 feet,  the 
base  will  sustain  a pressure  of  2 x 3 x 62.3  lbs.  = 
373.8  lbs. 

117.  Pressure  on  the  Side  of  a Vessel. — The  press- 
ure on  any  side  of  a vessel  containing  a liquid  is  equal 
to  the  weight  of  the  volume  of  liquid  obtained  by  mul- 
tiplying the  area  of  that  side  by  the  vertical  distance 
from  the  middle  of  the  side  to  the  surface  of  the  liquid. 

If  the  rectangular  side  of  a vessel  is  two  feet  long,  and 
the  vessel  be  filled  two  feet  deep  with  water,  then  the 


HYDROSTATICS. 


103 


area  of  the  side  covered  by  water  — 2 x 2 = 4 sq.  feet ; 
the  distance  from  the  centre  of  gravity,  which  will  be  at 
the  middle  of  the  side,  is  one  foot.  Then  the  pressure 
on  the  side  = 4 sq.  ft.  x 1 ft.  = 4 cubic  feet.  But  the 
weight  of  4 cubic  feet  of  water  = 4 x 62.3  = 249.2  lbs., 
which  equals  the  pressure  on  the  side. 

118.  Upward  Pressure. — The  upward  pressure  at 
any  part  of  a liquid  mass  is  equal  to  the  downward 
pressure  at  that  part. 

Experiment. — Take  a glass  cliimney,  B,  Fig.  44,  with  straight  sides, 
and  cut  a disc,  c d,  of  mica,  such  as  is  used  for  tlie  doors  of  stoves,  large 
enough  to  cover  the  base  of  the  chimney.  Attach  a string  to  the 
middle  of  the  disc.  Now,  placing  the  disc  so  as  to 
cover  the  smoother  end  of  the  chimney,  and  holding 
the  disc  in  its  place  by  the  string,  push  the  chimney 
vertically  downwards  in  a vessel,  A.  filled  with 
water.  When  some  little  depth  has  heeu  reached, 
the  string  need  no  longer  be  held,  as  the  upward 
pressure  caused  by  the  water  in  A endeavoring  to 
flow  into  B will  hold  the  disc  in  its  place. 

To  find  the  amount  of  this  upward  pressure,  pour 
water  carefully  into  the  chimney  until  the  mica  disc 
falls  off.  This  will  he  found  to  take  place  when  the 
level  of  the  water  inside  the  chimney  is  the  same  as  Fig- 44,— Upward 
that  on  the  outside.  At  this  moment,  the  downward 
pressure  caused  by  the  water  in  B trying  to  flow  out,  is  equal  to  the 
upward  pressure  caused  by  that  in  A trying  to  flow  in.  Since  the 
sides  of  the  chimney  are  vertical,  the  downward  pressure  is  equal  to 
the  weight  of  the  water  in  B ; hence  the  upward  pressure  on  c rf  is 
equal  to  the  weight  of  a column  of  water  whose  base  is  c d,  and  whose 
height  is  the  depth  of  the  liquid  above  cd. 

Caution. — The  end  of  the  chimney  must  be  quite  even  and  flat.  If 
mica  cannot  be  obtained,  a thin,  smooth  sheet  of  metal  may  be  used. 
If  much  difficulty  be  experienced  by  water  leaking  in,  the  end  of  the 
chimney  may  be  slightly  greased  with  tallow. 

119.  Surface  of  Liquids  at  Rest.— When,  a liquid 
is  at  rest,  its  upper  surface  is  everywhere  level  or  hori- 
zontal ; for,  were  it  higher  at  one  point  than  at  another, 


104 


NATURAL  PHILOSOPHY. 


the  greater  pressure  so  produced  would  press  it  up  at 
the  lower  points  until  the  whole  surface  became  level. 

It  is  only  the  surface  of  a comparatively  small  ex- 
tent of  water  that  is  horizontal.  Large  masses  like 
the  ocean  are  curved.  Lor  any  mass  of  water  to  he  in 
equilibrium,  all  parts  of  its  surface  must  be  at  equal 
distances  from  the  earth’s  centre. 

120.  Equilibrium  of  Liquids  in  Communicating 
Vessels.  — A liquid  in  communicating  vessels  is  in 
equilibrium  when  it  stands  at  the  same  height  or  level 
in  all  the  vessels;  for,  were  it  higher  in  one  vessel  than 
in  another,  the  greater  pressure  so  produced  would 
cause  it  to  mount  in  the  other  vessels  until  it  stood  at 
the  same  height.  The  water  rises  out  of  the  water- 
pipes  in  the  streets  and  fills  the  pipes  in  the  houses, 
because  the  level  of  the  water  in  the  basins,  or  reser- 
voirs in  which  it  is  stored,  is  equal  to  or  greater  than 
the  height  of  the  houses. 


In  artesian  wells,  the  water  is  forced  up  from  great 
depths  by  the  pressure  of  water  contained  between 
two  curved  impervious  strata. 


EYDROSTA  TICS. 


105 


When  two  liquids  of  different  densities  are  placed 
in  communicating  vessels,  they  Avill  be  in  equilibrium 
Avhen  the  level  of  tlie  denser  liquid  is  lower  than  the 
level  of  the  other  liquid.  The  heights  of  the  liquid 
columns  will  be  inversely  as  their  densities. 

121.  Bodies  Immersed  in  Liquids.  Buoyancy. — 

Bodies  immersed  in  a liquid  weiyli  less  than  they  do  in 
air.  They  lose  as  much  of  their  weight  as  the  weight 
of  the  liquid  they  displace.  This  important  principle 
was  discovered  by  Archimedes. 

Suppose,  for  example,  the  cube,  ahcd,  Fig. 46, be  com- 
pletely immersed  in  water ; then  the  pressures  on  the 
opposite  sides  being  equal,  neutralize  each  other.  The 
downward  pressure  is  equal  to  the 
weight  of  a column  whose  base  is  the 
top  of  the  cube,  and  whose  height 
is  e of ; the  upward  pressure  is  equal 
to  the  weight  of  a liquid  column 
whose  base  is  the  bottom  of  the 
cube,  and  whose  height  is  e c.  The 
upward  pressure,  therefore,  exceeds  • Fig.  46  — Cause  of 
the  downward  pressure  by  a weight  Buoyancy, 

of  water  equal  to  the  volume  of  the  cube. 

This  excess  of  upward  pressure  being  exerted  in 
a direction  opposite  to  that  of  the  force  of  gravity, 
causes  the  body  to  lose  as  much  of  its  Aveight  as  the 
weight  of  the  water  displaced  ; this  effect  is  called 
buoyancy. 

122.  Experimental  Proof  of  the  Principle  of 
Archimedes. — The  correctness  of  the  principle  an- 
nounced by  Archimedes  may  be  demonstrated  by  means 
of  the  apparatus  shown  in  Fig.  47.  A closed  cylinder, 
A,  of  such  a size  as  to  exactly  fill  the  hollow  cylinder. 


106 


NATURAL  PHILOSOPHY. 


A,  is  suspended  below  it,  and  tbe  two  attached  to  the  I 
pan  of  a balance,  and  exactly  counterpoised  in  air  by 
weights  at  C.  This  being  done,  the  ' 
cylinder,  B,  is  completely  inimersed 
in  water,  as  shown  in  the  figure.  The 
buoyancy  of  the  water  on  B causes 
it  to  lose  weight,  as  is  shown  b\' 
the  other  pan  of  the  balance  falling. 

If  now  the  cylinder,  J.,  be  exactly 
filled  with  water,  equilibrium  will 
be  restored.  The  weight  lost  by 

Fig.  47.  — Principle  of  i?,  therefore,  is  equal  to  the  weight 
Archimedes.  ^ Volume  of  Water  which  will 

just  fill  A\  but  this  volume  is  tbe  same  as  that  dis- 
placed by  i?,  therefore  the  weight  lost  by  B is  equal 
to  the  weight  of  the  water  it  displaces. 

123.  Floating  Bodies,^ — A body  placed  in  a liquid,  f 
will  float  if  it  displaces  a bulk  of  liquid  equal  to  its  : 
weight,  because  then  the  buoyancy  of  the  liquid  holds 
the  bod}'-  up  with  the  same  force  that  gravity  pulls  it 
down. 

Buoyancy  acts  at  a point  called  the  centre  of  buoy- 
ancy., Avhich  is  situated  at  the  centre  of  gravity  of  the 
displaced  liquid. 


124:.  Equilibrium  of  Floating  Bodies. — A floating 
body  will  be  in  equilibrium  when  the  centres  of  gravity 

and  buoyancy,  G and 
0,  are  in  the  same  ver- 
tical line.  Thus,inFig. 
48,  the  boat  at  A is  in 
equilibrium,  since  the 

Fig.  48.-Eqnilibrinm  of  Floating  Bodies. 

buoyancy  are  equal  and  directly  opposed  to  each  other ; | 


i 


HYDROSTATICS. 


107 


Fig.  49.  — Stable  Equilib- 
rium. 


but  if  tbe  boat  is  moved  into  the  position  represented 
at  B,  it  is  no  longer  in  equilibrium,  since  gravity 
and  buoyancy  are  no  longer  directly  opposed  to  each 
other,  but  tend  to  turn  the  boat  around,  until  they  are 
both  in  the  same  vertical  line. 

A floating  body  will  be  in  stable  equilibrium  when 
the  centre  of  buoyancy  is  above 
the  centre  of  gravity,  or  when  the 
centre  of  gravity  is  as  low  as  it 
can  get,  as,  for  example,  in  Fig.  49, 
where  the  centre  of  gravity  is  as 
low  as  it  can  be. 

A floating  body  is  in  unstable 
equilibrium  when  the  centre  of  buoyancy  is  below  the 
centre  of  gravity,  as  is  seen  in  Fig.  50. 

A floating  body  will  be  in  neutral 
equilibrium  when  the  relative  positions 
of  the  centres  of  gravity  and  buoyancy 
are  not  affected  by  any  movement  of  the 
body.  A sphere  floating  in  a liquid,  as 
shown  in  Fig.  51,  would  be  in  neutral 
equilibrium. 

In  the  above  examples,  it  will  be  no- 
ticed that  the  centre  of  buoyancy  is  the  point  of  sup- 
port of  the  floating  body,  and  bears  the 
same  relation  to  the  centre  of  gravity 
as  does  the  point  of  support  of  bodies 
which  can  move  freely  on  an  axis,  as 
will  be  seen  by  comparing  Figs.  49,  50, 
and  51  with  Figs.  28,  29,  and  30. 

The  equilibrium  of  a ship  or  boat  is  more  stable  as 
its  centre  of  gravity  is  lower.  When  a ship  is  not 
heavily  laden,  ballast  is  put  in  the  lower  part  of  the 
vessel  in  order  to  lower  the  centre  of  gravity. 


Fig.  50. — Unstable 
Eqnilibrinm. 


Fig.  51.— Neutral 
Equilibrium. 


108 


NATURAL  PIIILOSOniY. 


125.  Specific  Gravity. — By  the  specific /jravity  of  a 
hody^  we  mean  the  weight  of  that  body  as  compared 
with  the  weight  of  an  equal  bulk  of  some  other  body 
taken  as  the  standard  of  comparison. 

In  determining  the  specific  gravity  of  solids  and 
liquids,  we  compare  their  weight  with  the  weight  of 
an  equal  bulk  of  water ; and  in  determining  that  of 
gases  and  vapors,  with  the  weight  of  an  equal  bulk 
of  air. 

Rule. — To  determine  the  specific  gravity  of  a solid  or  a 
liquid^  divide  the  weight  of  the  body  in  air  by  the  v:eight 
of  an  equal  volume  of  water. 

The  same  rule  may  be  expressed  as  follows,  viz. : 

or  Sp  Gr  ^ Weight  of  body  in  air. 

W'  Weight  of  equal  hulk  of  water. 

Fig.  52.— A general  Formula  for  Specific  Gravity. 

To  determine  the  specific  gravity  of  any  gas,  we  di- 
vide the  weight  of  the  gas  by  the  weight  of  an  equal 
volume  of  air,  or  other  gas,  taken  as  a standard. 

In  order,  therefore,  to  determine  the  specific  gravity 
of  any  body,  it  is  only  necessary  to  ascertain  the  weight 
of  the  body  and  the  weight  of  an  equal  bulk  of  water 
or  gas. 

Suppose,  for  example,  we  wisli  to  determine  the 
specific  gravity  of  a piece  of  iron.  We  first  find 
the  weight  of  the  iron  in  air,  Avhich  Ave  A\ill  suppose 
to  be  778  grains.  We  then  find  the  AAmight  of  an  equal 
bulk  of  Avater,  which  Avill  be,  say  100  grains.  The 
specific  gravity  of  the  iron,  therefore,  is  7.78,  or, 

in  other  words,  the  iron  is  7.78  times  heavier  than 
Avatcr. 


HYDRO  ST  A TICS. 


109 


126.  Methods  of  Obtaining  Specific  Gravity. — 
By  the  Balance.  For  Solids.  Since  a body  im- 
mersed in  water  loses  exactly  as  mucli  weight  as  the 
weight  of  the  water  it  displaces,  it  is  easy  to  obtain 
the  specific  gravit}^  by  this  method.  For,  attach  the 
body  to  a string  tied  to  one  of  the  pans  of  a balance ; 
exactly  balance  the  body  by  adding  weights  to  the 
other  pan;  these  weights  will  give  us  the  value  of  IF, 
that  is,  the  weight  of  the  solid  in  air.  Then,  while 
the  solid  is  still  attached  to  the  balance,  immerse  it 
in  water,  and  find  how  much  weight  it  loses.  This 
weight  will  give  us  IF',  or  the  weight  of  a bulk  of 
water  equal  to  that  of  the  solid.  Then  IF  divided  by 
IF'  will  give  the  specific  gravity  of  the  solid.  Sup- 
pose, for  example,  a solid  weighs  in  air  200  grains, 
and  loses  in  water  150  grains,  then  its  specific  gravity 
= Hg  = 1.33. 

127.  For  Solids  Lighter  than  Water. — If  the  solid 
is  lighter  than  Avater,  attach  it  to  another  solid,  as,  for 
example,  a piece  of  copper,  heavy  enough  to  sink  it  in 
water.  Find  how  much  weight  the  two  lose  when  im- 
mersed in  water.  Find  how  much  weight  the  copper 
itself  loses  when  immersed  in  water ; then  the  differ- 
ence between  this  weight  and  the  weight  that  both 
lose  will  give  the  weight  the  light  body  loses  in 
water.  Divide  the  Aveight  of  the  light  body  in  air 
by  the  Aveight  it  loses  in  Avater,  and  the  quotient  Avill 
be  the  specific  graAuty.  Suppose  the  lighter  solid 
weighs  6 grains,  and  that  Avhen  both  are  immersed  in 
AA'ater  they  lose  10  grains,  and  that  the  heavier  solid 
loses  Avhen  immersed  1 grain.  Then  the  lighter  solid 
must  lose  9 grains,  and  its  specific  gravity  must  equal 
I or  .66. 

10 


no 


NATURAL  PHILOSOPHY. 


128.  For  Liquids.  — A closed  bulb,  A,  Fig.  53, 

partly  filled  with  mercury  or  other  heavy  substance, 

is  attached  to  a string,  and  suspended  from  one  of  the 

pans  of  a balance.  First  find  the  weight  the  bulb  loses 

when  immersed  in  the  liquid  whose  specific 

gravity  is  desired.  This  will  be  the  weight  of  a 

quantity  of  liquid  equal  to  the  volume  of  the 

bulb,  D ; then  find  the  weight  the  bulb,  A,  loses 

when  immersed  in  water ; this  will  be  the  weight 

of  a volume  of  water  equal  to  that  of  the  bulb. 

Then  divide  the  weight  the  bulb  loses  in  the 

j)  liquid,  whose  specific  gravity  is  desired,  b}"  the 

Fig. 63.  weight  it  loses  in  water,  and  the  quotient  will 
Specific-  , ^ 

Gravity  be  the  specific  gravity.  Thus,  suppose  the 

Bulb.  J)^  loses  184  grains  when  immersed  in 

sulphuric  acid,  and  100  grains  when  immersed  in 
water  ; then  = 1.84,  the  specific  gravity  of  the 
sulphuric  acid. 


Ui 


129.  By  the  Specific-Gravity  Bottle. — The  spe- 
cific gravity  of  a liquid  may  be  very  conveniently  found 
by  means  of  a bottle.  The  bottle  is  first  weighed  when 
empty.  It  is  then  filled  with  the  liquid,  sa}^  milk, 
whose  specific  gravity  is  desired,  and  again  weighed : 
this  weight,  less  the  weight  of  the  bottle,  will  give  the 
weight  of  a quantity  of  milk  that  will  exactly  fill  the 
bottle ; the  bottle  is  then  emptied  of  milk  and  filled 
with  water,  and  again  weighed.  This  weight,  less  the 
weight  of  the  bottle,  will  give  the  Aveight  of  a quan- 
tity of  water  that  will  exactly  fill  the  bottle ; then  the 
weight  of  the  milk  that  Avill  exactly  fill  the  bottle, 
divided  by  the  weight  of  Avater  that  Avill  exactly  fill 
the  bottle,  Avill  give  the  specific  graAdty  of  the  milk. 
Thus,  suppose  the  bottle,  Avhen  empty,  Aveighs  300 


I 

I 


I 


HYDROSTATICS. 


Ill 


grains,  and  when  filled  Avith  milk,  1326  grains,  and 
when  filled  with  water,  1300  grains ; then  1326  — 300 
= 1026  grains,  the  weight  of  the  milk,  and  1300  — 
300  = 1000  grains,  the  weight  of  the  water,  and 
= 1.026,  the  specific  gravity  of  the  milk. 

130.  By  the  Hydrometer.  — The  specific  gravity 
of  a liquid  may  be  very  easily  found  by  the  use  of 
floating  instruments  called  hydrometers. 

One  form  of  this  instrument  is  seen  in  Fig.  54. 
It  is  made  of  glass,  and  is  hollow  except  at  its  lower 
end,  which  contains  mercury,  so  as  to  make  it  float 
upright  when  placed  in  any  liquid.  It 
is  evident,  that  when  the  instrument  is 
placed  in  any  liquid,  it  will  sink  until  it 
displaces  a bulk  of  liquid  equal  in  weight 
to  its  own  weight.  But  the  denser  the  liq- 
uid, the  less  deep  it  will  sink,  since  the 
less  Avill  be  the  bulk  of  liquid  required  to 
equal  in  weight  the  weight  of  the  instru- 
ment. 

We  determine  the  specific  gravity  of  Pig.  54.  — Hy- 
any  liquid  in  Avhich  the  hydrometer  is  drometer. 
placed  by  comparing  the  distance  to  which  the  instru- 
ment sinks  Avhen  placed  in  water  with  the  distance 
to  Avhich  it  sinks  Avhen  placed  in  the  liquid  Avhose 
specific  gravity  is  desired.  A scale,  already  calcu- 
lated, marked  on  the  tube  generally  gives  the  specific 
gravity. 

Instruments  of  this  kind,  when  used  to  determine 
the  specific  gravity  of  milk,  are  called  lactometers ; and 
of  alcohol,  alcoholometers. 

In  the  folloAving  table,  the  specific  gravity  of  a fcAV 
common  substances  Avill  be  found  : 


112 


NATURAL  PHILOSOPHY. 


Solids. 


Iron 

7.78 

Zinc 

7.19 

Lead 

11.35 

Copper 

— 

8.90 

Silver 

10.47 

Gold 

= 

19.30 

Platinum 

= 

22.06 

Granite 

2.75 

Ice  = 

.87 

Cork  = 

.24 

Liquids. 

Mercury  = 

13.50 

Sulphuric  acid  = 

1.84 

Milk  = 

1.026 

Ocean  water  = 

1.026 

Alcohol  = 

.792 

Ether  = 

.715 

I 


Since  the  weight  of  a cubic  foot  of  water  = 62.3  Ihs., 
if  we  know  the  specific  gravity  of  any  substance,  we 
can  easily  calculate  the  weight  of  a cubic  foot  of  that 
substance ; thus,  the  sp.  gr.  of  gold  = 19.30 ; then  a 
cubic  foot  of  gold  = 19.30  x 62.3  = 1202.39  lbs.;  so, 
also,  if  we  know  the  weight  of  a body  and  its  sp.  gr., 
we  can  calculate  its  volume.  Thus,  what  is  the  vmlume 
of  100  lbs.  of  gold  ? Since  one  cubic  foot  of  gold  has 
a weight  of  1202.39  lbs.,  100  lbs.  must  occupy  the 
1 2 01^3  y ^ cubic  foot. 


Syllabus. 

Hydrodynamics  treats  of  the  conditions  of  rest  and  motion  in  fluids. 
It  includes  Hj’^drostatics,  Hydraulics,  and  Pneumatics.  H5’drostatics 
treats  of  liquids  at  rest;  Hydraulics,  of  liquids  in  motion  ; and  Pneu- 
matics, of  gases  either  at  rest  or  in  motion. 

Liquids  are  almost  incompressible.  They  transmit  in  all  directions, 
and  without  sensible  loss  of  intensity,  the  pressure  exerted  on  any 
portion  of  their  mass. 

The  total  pressure  sustained  by  any  surface  immersed  in  a liquid  is 
proportional  to  its  area;  this  gives  us  another  mechanical  power,  as  is 
seen  in  the  hydrostatic  press. 

The  pressure  caused  by  the  weight  of  a liquid  increases  with  the 
depth  and  density  of  the  liquid. 

Since  liquids  exert  pressure  equally  in  all  directions,  the  downward, 
upward,  and  lateral  pressures  at  any  point  must  all  he  equal  to  one 
another. 


SYLLABUS. 


113 


The  pressure  on  the  base  of  a vessel  filled  with  liquid  is  sometimes 
greater  and  sometimes  less  than  tlie  weight  of  the  liquid  in  the  vessel. 
To  obtain  the  pressure  on  the  base,  calculate  the  weight  of  a column  of 
liquid  whose  base  is  the  base  of  the  vessel,  and  whose  height  is  the  ver- 
tical distance  from  the  middle  of  the  base  to  the  surface  of  the  liquid. 

To  determine  the  pressure  on  the  side  of  a vessel,  calculate  the  weight 
of  a volume  of  the  liquid  equal  to  the  area  of  the  side  multiplied  by 
the  vertical  distance  from  the  middle  of  the  side  to  the  surface. 

The  upward  pressure  at  any  point  is  equal  to  the  downward  press- 
ure at  that  point. 

The  upper  surface  of  a comparatively  small  extent  of  water  is  every- 
where level  or  horizontal.  The  surface  of  a large  body  of  water  like 
the  ocean  is  curved. 

When  a liquid  is  placed  in  communicating  vessels,  it  will  be  in  equi- 
librium only  when  its  surface  is  at  the  same  level  in  all  the  vessels. 

Bodies  immersed  in  liquids  lose  as  much  of  their  weight  as  the 
weight  of  the  liquid  they  displace,  because  the  excess  of  upward  press- 
ure which  buoys  them  up  is  equal  to  the  weight  of  the  liquid  displaced. 

A body  floats  when  the  weight  of  the  liquid  it  displaces  is  equal  to 
its  own  weight,  since  the  force  which  pulls  it  down  is  then  just  equal 
to  the  force  which  holds  it  up. 

A floating  body  is  in  equilibrium  when  the  centre  of  buoyancy 
and  the  centre  of  gravity  are  in  the  same  vertical  line.  The  centre 
of  buoyancy  is  the  centre  of  gravity  of  the  displaced  liquid. 

If  the  centre  of  gravity  of  a floating  body  in  equilibrium  is  as  low 
as  it  can  get,  or  if  the  centre  of  buoyancy  is  above  the  centre  of  grav- 
ity, the  equilibrium  will  be  stable.  If  the  centre  of  buoyancy  is  be- 
low the  centre  of  gravity,  the  equilibrium  will  be  unstable.  If  no 
motion  of  the  body  can  alter  the  relative  positions  of  the  centres  of 
buoyancy  and  gravity,  the  equilibrium  will  be  neutral. 

The  specific  gravity  of  a body  is  the  weiglit  of  the  body  in  air 
divided  by  the  weight  of  an  equal  bulk  of  some  other  substance.  To 
find  the  specific  gravity  of  a solid  or  liquid,  divide  the  weight  of  the 
body  in  air  by  the  weight  of  an  equal  bulk  of  water.  To  find  the 
specific  gravity  of  a gas,  divide  the  weight  of  the  gas  by  the  weight  of 
an  equal  bulk  of  air. 

The  specific  gravity  of  a body  may  be  found  by  the  balance,  by  the 
specific-gravity  bottle,  or  by  the  hydrometer. 

The  weight  of  any  volume  of  a substance  is  equal  to  its  specific 
gravity  multiplied  by  the  weight  of  an  equal  volume  of  water. 

' 10*  H 


114 


NATURAL  PHILOSOPHY. 


Questions  for  Review. 

Define  Hydrodynamics ; Hydrostatics  ; Hydraulics ; Pneumatics. 
How  does  the  compressibility  of  liquids  compare  wdth  that  of  solids? 

State  the  law  for  the  transmission  of  pres.=ure  in  liquids. 

Describe  the  construction  of  the  hydrostatic  press. 

What  relation  exists  between  the  pressure  caused  by  the  weight  of 
a liquid  and  its  depth  ? Between  the  pressure  caused  by  the  weight 
of  a liquid  and  its  density  ? 

Why  should  the  upward,  downward,  and  lateral  pressures  at  any 
point  of  a liquid  he  equal  to  one  another  ? 

Give  an  example  of  a vessel  in  which  the  pressure  on  the  base  of 
the  liquid  arising  from  the  weight  is  greater  than  the  whole  weight  of 
the  liquid.  Give  an  example  in  which  this  pressure  is  less  than  the 
whole  weight  of  the  liquid. 

State  the  rules  for  calculating  the  pres.sure  on  the  base  of  a vessel 
filled  with  liquid  ; on  the  vertical  wall  of  a vessel ; the  upward  pressure 
at  any  part  of  the  liquid. 

Describe  an  experiment  by  which  the  amount  of  the  upward  press- 
ure of  a liquid  may  be  determined. 

What  will  be  the  shape  of  the  surface  of  a comparative!}'  small  ex- 
tent of  a liquid  at  rest?  tWiy  ? 

When  will  a liquid  placed  in  communicating  vessels  be  in  equi- 
librium ? 

What  do  you  understand  by  the  buoyancy  of  a liquid  ? How  much 
weight  will  a body  lose  when  immersed  in  a liquid? 

When  will  a body  float  in  a liquid  ? When  will  a floating  body  be 
in  equilibrium  ? 

What  is  meant  by  the  specific  gravity  of  a body  ? With  what  do  we 
generally  compare  the  weight  of  solids  and  liquids?  Of  gases? 

State  the  general  rule  for  obtaining  the  specific  gravity  of  a body. 
State  the  general  formula. 

Describe  the  method  of  obtaining  the  specific  gravity  of  a solid  by 
means  of  a balance,  1st.  When  the  body  is  heavier  than  water  ; 2d. 
When  the  body  is  lighter  than  water. 

Describe  the  method  of  obtaining  the  specific  gravity  of  a liquid  by 
means  of  a balance.  By  means  of  a specific-gravity  bottle.  By  means 
of  a hydrometer. 


1 


CHAPTER  IT 

HYDRAULICS. 

131.  Hydraulics  treats  of  liquids  in  motion.  It 
studies  tlie  flow  and  elevation  of  liquids,  and  the  ma- 
chines for  moving  liquids,  or  to  be  moved  by  them. 

132.  Pressures  on  a Vessel  Containing  Liquid. — 

The  walls  of  a vessel  filled  with  liquid  and  exposed  to 
the  air  are  subjected  to  two  pressures,  viz. ; 

1st.  The  pressure  of  the  liquid  from  within  out- 
wards. 

2d.  The  pressure  of  the  air  from  without  inwards. 

If  a hole  be  pierced  in  the  side  of  a vessel  containing 
liquid,  the  liquid  will  escape  only  when  the  pressure 
from  within  outwards  is  greater  than  the  atmospheric 
pressure.  If  the  vessel  be  open  to  the  air  at  the  top, 
the  pressure  of  the  air  tends  to  force  the  liquid  out 
with  the  same  force  that  it  tends  to  keep  it  in.  In 
this  case,  therefore,  the  liquid  tends  to  run  out  with  a 
force  equal  to  the  pressure  caused  by  the  depth  of  the 
liquid  above  the  opening. 

Water  flowing  from  a narrow-necked  bottle  does  not  escape  in  a 
steady  stream,  but  at  more  or  less  regular  intervals  partially  stops 
flowing,  when  a few  bubbles  of  air  enter  the  neck  of  the  bottle  with  a 
gurgling  sound,  and  the  full  flow  again  begins.  The  partial  stoppages 
are  due  to  the  pressure  of  the  atmosphere,  which  forces  bubbles  of 

115 


116 


NATURAL  PHILOSOPHY. 


air  into  the  bottle  against  the  pressure  of  the  escaping  liquid.  The  air 
thus  forced  into  the  bottle  occupies  the  space  left  by  the  liquid  which 
has  escaped.  After  a certain  quantity  of  liquid  has  escaped,  the  piress- 
ure  of  the  air  against  the  mouth  of  the  bottle  is  greater  than  the 
pressure  forcing  the  liquid  out.  Some  air  then  enters,  and  more  liquid 
escapes.  Were  a hole  made  in  the  bottom  of  the  bottle,  the  liquid 
would  escape  in  a steady  stream. 

133.  Velocity  of  Escape.  — If  holes  he  bored  in 
the  side  of  an  open  barrel  filled  with  water,  it  will  be 
found,  as  might  be  supposed,  that  the  tvater  will  flo\y 
most  rapidly  out  of  the  hole  tvhich  is  nearest  the  bot- 
tom. The  water  flows  out  of  the  openings  solely  by 
reason  of  the  pressure  of  the  liquid  at  the  opening ; 
and  as  the  amount  of  pressure  exerted  by  a liquid 
increases  with  the  depth,  the  greater  the  depth  of  the 
hole  below  the  surface  the  greater  the  velocity  Avith 
which  the  liquid  escapes. 

If  the  holes  in  the  side  of  the  barrel  be  all  of  the  same  size,  it  can 
be  proved  that  the  liquid  escapes  the  most  rapidly  from  the  lowest 
hole ; for,  if  a vessel  be  held  for  one  minute  before  each  of  the  holes, 
so  as  to  catch  all  the  liquid  which  escapes  from  that  hole  in  one  minute, 
it  will  be  found  that  the  hole  nearest  the  bottom  of  the  barrel  will 
discharge  more  than  any  of  the  others ; the  velocity  of  escape  of  the 
liquid  from  this  hole  must,  therefore,  be  greater  than  from  any  other. 

An  opening  in  tlie  side  of  a vessel  tbrougli  wbich. 
liquid  escapes  is  called  an  orifice.  The  \mrtical  dis- 
tance from  the  middle  of  the  orifice  to  the  surface  of 
the  liquid  is  called  the  head.  The  amount  of  liquid 
Avhich  iloAVS  out  of  an  orifice  in  a given  time  is  called 
the  flow. 

134.  Rule  for  Calculating  the  Velocity  of  Escape. 

— W e have  seen  that  the  Amlocity  of  escape  of  a liq-  | 
uid  increases  Avith  the  depth  of  the  orifice  below  the  j 
surface,  that  is,  Avith  the  head.  Torricelli,  an  Italian 
philosopher,  discoAmred  that  the  A'elocit}'  Avith  Avhich 


HYDRAULICS. 


117 


the  liquid  escapes  is  exactly  the  same  as  the  velocity 
it  would  acquire  in  falling  in  an  empty  space  through 
the  distance  of  the  head.  This  fact  is  expressed  ap- 
proximately by  the  following  simple  formula,  viz. : V 
= 8 VH,  when  V = the  velocity  of  escape  per  second, 
and  H = the  head. 

The  rule  may  be  expressed  as  follows,  viz. : The  ve- 
locity in  feet  per  second  with  which  a liquid  escapes 
from  an  orifice.^  is  equal  to  eight  times  Ulc  square  root 
of  the  head. 

Suppose,  for  example,  that  an  orifice  was  four  feet  below  the  surface 
of  a liquid,  then  the  velocity  of  escape  would  be8Xl^'i  = 8X2  = 
16  feet  per  second. 

135.  Method  of  Ascertaining  the  Flow. — To  as- 
certain the  flow\  or  the  amount  of  liquid  escaping  from 
an  orifice  in  a given  time.,  multiply  the  velocity  of  escape 
hy  the  area  of  the  orifice.  Since  the  velocity  obtained 
from  the  preceding  formula  tvill  be  in  feet,  if  the  area 
of  the  orifice  be  in  square  inches  the  velocity  must 
first  be  reduced  to  inches ; the  product  will  then  give 
the  volume  of  the  flow  in  cubic  inches  per  second. 

The  product  of  the  area  of  the  orifice  and  the  velocity  must  give  the 
volume  of  the  discharge,  as  the  following  simple  reasoning  will  show. 
Suppose  the  area  of  the  orifice  to  be  one  square  inch,  and  the  flow 
twelve  inches  per  second  ; then  in  one  second  a mass  of  water  one  inch 
thick  and  twelve  inches  long  would  flow  out  from  the  orifice,  the  vol- 
ume of  which  would  of  course  be  12  cubic  inches. 

Since  the  velocity  of  escape  from  any  vessel  varies 
AV'ith  the  head,  if  this  decreases  as  the  liquid  runs  out, 
the  quantity  discharged  from  any  given  orifice  must  be 
greater  during  the  first  second  than  during  the  second, 
and  greater  during  the  second  second  than  during  the 
third,  and  so  on.  AVe  cannot,  therefore,  ascertain  the 
quantity  discharged  in,  say  60  seconds,  by  multiplying 


118 


NATURAL  PHILOSOPHY. 


the  quantity  discharged  during  the  first  second  by  60, 
unless  the  head  is  kept  constant  by  allowing  w'ater  to 
run  into  the  vessel  as  fast  as  it  runs  out. 

The  flow,  as  calculated  by  the  preceding  rule,  will 
be  found  in  practice  to  be  greater  than  the  amount 
which  actually  escapes.  This  is  because  the  issuing 
stream  contracts  shortly  after  leaving  the  orifice.  The 
amount  which  actually  escapes  is  about  equal  to  two- 
thirds  of  the  calculated  amount. 

136.  The  Flow  of  Liquids  through  Horizontal 

Pipes. — When  a liquid  flows  through  long  pipes,  a.<j, 
for  example,  through  the  water-pipes  of  a city,  the 
friction  of  the  water  against  the  sides  of  the  pipes 
greatly  diminishes  the  velocity.  This  is  especially  the 
case  at  the  bends  of  the  pipe,  where  the  liquid  is  forced  I 

to  suddenly  change  the  direction  of  its  motion.  It  is  ' 

on  account  of  this  decrease  in  the  velocity,  that  the  ! 
water  must  be  put  under  considerable  pressure  in  order 

to  cause  a sufficient  volume  of  liquid  to  flow  through  ! 
the  pipes. 

137.  The  Velocity  of  Rivers.  — The  velocity  of 
flow  of  a river  depends  on  the  inclination  of  its  bed  or  | 
channel,  that  is,  on  the  difference  of  level  between  the 
source  and  the  mouth.  This  difference  of  level  is  the 
head  under  which  the  water  escapes,  but  the  velocity 
that  would  be  due  to  such  a head  is  enormously  greater 
than  that  which  the  river  actualh*  possesses,  since  the 
friction  of  the  water  against  the  air  and  the  channel 
decreases  its  velocity.  The  greater  the  volume  of 
water  in  a river,  and  the  greater  the  inclination,  the 
greater  the  velocity. 

The  velocity  of  the  water  at  the  surface  is  somewhat 
less  than  that  at  some  distance  below  the  surface.  The 


HYDRAULICS. 


119 


surface  velocity  varies  from  about  two  to  four  miles 
per  hour. 

138.  Vertical  Jets.  — Wben  a jet  of  water  escapes 
vertically  upwards,  it  should  rise  as  high  as  the  level 
of  the  liquid  in  the  reservoir,  because  the  velocity 
with  which  it  escapes  is  equal  to 
that  it  would  acquire  in  falling 
through  a space  equal  to  the  ver- 
tical distance  from  the  orifice  to  the 
level  of  the  liquid,  and  this  velocity 
should,  therefore,  carry  it  upwards 
through  the  same  height. 

The  jet,  however,  does  not  act- 
ually rise  quite  as  high  as  the  level 
of  the  water  in  the  reservoir,  be- 
cause, 1st.  Its  velocity  is  diminished  by  friction  against 
the  sides  of  the  orifice ; 2d.  Some  of  its  motion  is  lost 
in  pushing  the  air  out  of  the  way ; and,  3d.  The  water 
which  is  falling  strikes  that  which  is  rising,  and  so 
decreases  the  velocity. 

139.  Water-Wheels. — The  moving  water  of  a 
river  represents  a considerable  amount  of  energ}^. 
We  may  utilize  this  energy  and  cause  it  to  do  work, 
by  making  the  moving  water  give  part  of  its  motion 
to  a water-wheel. 

The  principal  forms  of  water-wheels  are  the  tinder- 
shot.,  the  overshot,  the  hreast-voheel,  and  the  turbine. 

140.  The  Undershot-Wheel.  — In  the  undershot 
water-wheel,  the  water  strikes  near  the  bottom  of  the 
wheel,  against  a number  of  flat  boards  called  float- 
boards,  placed  as  shown  in  Fig.  56.  The  wheel  is 
moved  by  the  force  of  the  current. 


Fig.  55.—  Height  of  Jet. 


Fig.  57.— The  Overshot-Wheel. 

until  it  readies  the  lowest  point.  The  wheel  is  moved 
by  the  force  of  the  current  and  by  the  weight  of  the 
water  in  the  buckets,  since  the  side  of  the  wheel  that 
receives  the  water  is  heavier  than  the  opposite  side. 


120  NATURAL  PHILOSOPHY. 

If  the  stream  is  a tidal  one,  so  that  the  water  sometimes  flows  in 
one  direction  and  sometimes  m another,  the  float-boards  are  generally 


Fig.  56.—  The  Undershot- Wheel. 

placed  at  right  angles  to  the  rim  of  the  wheel;  when,  however,  the 
direction  of  the  stream  is  constant,  the  float-boards  are  inclined  at  an 
acute  angle  to  the  current,  in  which  case  the  water  acts  partly  by  its 
weight. 

141.  The  Overshot-Wheel. — In  the  overshot-wheel 
the  water  is  received  at  the  top  of  the  wheel  in  buckets, 
shaped  as  shown  in  Fig.  57,  so  as  to  retain  the  water 


HYDRAULICS. 


121 


The  overshot-wheel  is  applicable  when  the  amount  of  water  is 
small,  but  the  velocity  great.  The  undershot-wheel  is  used  when  the 
quantity  of  water  is  great,  and  its  velocity  comparatively  small. 

142.  The  Breast- Wheel. — In  the  breast-wheel  the 
water  is  received  on  the  wheel  at  or  near  the  level  of 
the  axis,  A,  as  shown  in  Fig.  58.  The  buckets  are 


placed  perpendicularly  to  the  circumference  of  the 
wheel,  and  are  arranged  so  as  to  hold  the  water  until 
they  reach  the  lowest  point,  Avhich  is  done  by  causing 
the  ends  of  the  buckets  to  move  near  the  curved  way 
down  Avhich  the  Avater  runs.  The  breast- Avheel  is 
turned  by  the  force  of  the  current  and  the  weight  of 
the  Avater. 

The  breast-wheel  is  an  economical  form  of  Avater-wheel,  and  is  es- 
pecially applicable  Avhen  the  volume  of  water  is  comparatively  large 
and  the  velocity  moderate. 

143.  Reaction  of  the  Escaping  Jet. — When  a jet 
of  liquid  is  escaping  from  an  orifice  in  the  side  of  a 
vessel,  it  Avill  produce  a pressure  Avhich,  if  the  vessel 
be  free  to  move,  Avill  cause  it  to  move  in  a direction 
opposite  to  that  in  Avhich  the  liquid  is  escaping.  Sup- 
pose, for  example,  the  vessel.  A,  Fig.  59,  has  at  its  loAAmr 
end  a horizontal  tube,  (7,  the  ends  of  Avhich  are  bent 
as  shown.  As  the  liquid  escapes  from  openings  at  the 
11 


122 


NATURAL  PHILOSOPHY. 


extremities  of  tliis  tube,  tbe  vessel  rotates  in  a direc- 
tion opposite  to  that  in  which  the  liquid  is  escaping. 

The  cause  is  as  follows ; 
Were  the  openings  at  the 
ends  of  the  tube  closed,  the 
pressure  on  anj  portions  of 
the  wall  that  are  directly  op- 
posite each  other  would  be 
equal,  and  bei  ng  exerted  in  op- 
posite directions  would  neu- 
tralize each  other.  But  when 
the  liquid  begins  to  run  out 
of  the  openings,  the  pressure 
at  these  points  is  removed, 
and  the  pressure  on  the  opposite  side  no  longer  being 
neutralized,  moves  the  vessel  round.  The  direction  in 
which  this  pressure  acts  is  showir  at  D.  The  effect  so  > 
produced  by  the  escape  of  the  liquid  is  sometimes  j 
called  the  reaction  of  the  escaping  jet.  i 


Fig.  59. — Reaction  Vase. 


14:4:.  The  Turbine  Water-Wheel 

water-Avheel,  advantage  is  taken  of 
the  reaction  of  the  escaping  jet. 


. — In  the  turbine 


i 


1 

1 


Fig.  60.— The  Turbine  Water-Wheel. 


The  figures  represent  one  form  of  turbine  in  per- 
spective and  in  horizontal  section.  This  form  of  wheel 


SYLLABUS. 


123 


is  submerged  in  water  in  such  a place  that  the  water 
can  readily  escape  from  the  wheel  after  it  has  given 
motion  to  it.  The  top  of  the  wheel  is  covered,  to  pro- 
tect it  from  the  direct  pressure  of  the  water.  The 
movable  part  of  the  wheel  is  seen  at  a a a a,  which 
is  attached  to  the  shaft,  A.  The  water  enters  below 
through  openings  between  the  fixed  curved  guides, 
(j  <j  (j  <j.  The  curve  of  the  guides  is  so  inclined  to  that 
of  the  buckets,  that  the  water  strikes  the  buckets  in 
the  most  advantageous  direction,  and  drives  the  wheel 
by  the  force  of  the  current,  and  partly  also  by  the 
weight  of  the  water  in  the  buckets;  on  running  out  of 
tlie  buckets  the  reaction  of  the  escaping  streams  also 
turns  the  wheel  in  the  same  direction  as  the  force  of 
the  current  and  the  weight  of  water  in  the  buckets. 
The  turbine  is,  as  might  be  supposed,  a very  econom- 
ical form  of  water-wheel.  After  it  has  escaped  from 
the  buckets,  the  dead  water as  it  is  called,  is  generally 
carried  oft  by  means  of  a sluice  connected  with  the 
lower  part  of  the  wheel. 


Syllabus. 

Hydraulics  treats  of  liquids  in  motion.  It  studies  the  flow  and  ele- 
vation of  liquids  and  the  construction  of  machines  for  moving  liquids 
or  moved  by  them. 

The  walls  of  a vessel  containing  liquid  sustain  two  pressures,  viz. : 
1st.  The  pressure  of  the  liquid  exerted  from  within  outward,  and,  2d, 
The  pressure  of  the  atmosphere  from  without  inward. 

When  an  opening  is  made  in  the  side  of  a vessel  containing  a liquid, 
the  liquid  will  escape,  provided  the  pressure  with  which  it  is  forced  out 
is  greater  than  that  of  the  atmosphere  tending  to  keep  it  in.  If  the 
vessel  be  open  at  the  top,  the  pressure  of  the  air  tending  to  force  the 
liquid  out  will  be  equal  to  the  pressure  tending  to  keep  it  in. 


124 


NATURAL  PniLOSOPHY. 


If  holes  be  made  in  the  side  of  an  open  barrel  filled  with  water,  the 
velocity  of  escape  will  be  greater  from  the  holes  near  the  bottom  of  the 
barrel  than  from  those  near  the  top. 

An  opening  in  a vessel  through  which  a liquid  escapes  is  called  an 
orifice.  The  vertical  distance  from  the  middle  of  the  orifice  to  the  sur- 
face of  the  liquid  is  called  the  head.  The  amount  of  liquid  which 
escapes  from  an  orifice  in  a given  time  is  called  the  flow. 

To  obtain  the  velocity  with  which  a liquid  escapes  from  an  orifice, 
multiply  the  square  root  of  the  head  by  eight.  The  product  will  give 
the  velocity  in  feet  per  second. 

To  obtain  the  amount  of  the  discharge,  or  the  flow,  multiply  the  area 
of  the  orifice  by  the  velocity  of  escape.  In  practice,  it  is  necessar}-  to 
diminish  the  quantity  so  obtained  by  about  one-third,  because  the 
stream  contracts  shortly  after  leaving  the  orifice. 

When  a liqnid  flows  through  a horizontal  pipe,  the  velocity  is  de- 
creased by  friction. 

The  velocity  of  a river  is  greatly  diminished  by  friction  against  the 
bottom  and  sides  of  the  channel,  and  against  the  air. 

There  are  three  causes  which  prevent  a jet  of  water  escaping  verti- 
cally upwards  from  rising  as  high  as  the  level  of  the  water  in  the  res- 
ervoir, viz. ; 1st.  The  friction  against  the  sides  of  the  orifice ; 2d.  The 
resistance  of  the  air  ; and  3d.  The  falling  water. 

The  moving  water  of  a river  represents  considerable  energy.  This 
energy  may  be  utilized  by  causing  the  moving  water  to  give  part  of 
its  motion  to  a water-wheel. 

In  the  undershot  water-wheel,  the  water  is  received  below,  on  float- 
boards.  This  wheel  is  driven  bj^  tbe  force  of  the  current. 

In  the  overshot-wheel,  the  water  is  received  above  by  buckets  con- 
structed so  as  to  retain  the  water  until  they  reach  the  lowest  point. 
This  wheel  is  driven  by  the  force  of  the  current  and  the  weight  of  the 
water. 

In  the  breast- wheel,  the  water  is  received  at  or  near  the  level  of  the 
axis  by  buckets  so  arranged  as  to  retain  the  water  until  they  reach  the 
lowest  point.  This  wheel  is  driven  by  the  force  of  the  stream  and  the 
weight  of  the  water. 

In  the  turbine,  the  water  is  directed  by  stationary  curved  guides,  so 
as  to  strike  the  buckets  in  the  most  advantageous  direction.  This 
wheel  is  driven  by  the  force  of  the  current  and  the  reaction  of  the 
water  as  it  escapes  from  the  buckets.  It  is  also  driven  to  some  extent 
by  the  weight  of  the  water  in  the  buckets.  The  turbine  is  therefore  a 
very  economical  form  of  water-wheel. 


QUESTIONS  FOR  REVIEW. 


]25 


Questions  for  Reviev/. 

Define  hydraulics.  How  does  hydraulics  dififer  from  hydrostatics  ? 

What  two  pressures  does  a vessel  filled  with  liquid  sustain  ? When 
will  a liquid  escape  from  an  orifice  in  a vessel  ? 

Explain  the  cause  of  the  intermittent  flow  of  water  from  a bottle 
with  a narrow  neck. 

Prove  that  the  velocity  of  escape  from  an  orifice  increases  with  the 
depth  of  the  orifice  below  the  surface. 

Define  orifice,  head,  and  flow. 

Give  the  rule  for  calculating  the  velocity  of  escape  of  a liquid  from 
an  orifice.  To  what  is  this  velocity  equal  ? 

Give  the  rule  for  obtaining  the  flow  or  the  quantity  of  liquid  dis- 
charged in  a given  time.  Why  is  the  amount  obtained  by  this  rule 
greater  than  the  actual  flow?  How  much  greater  is  it?  Why  must 
the  head  remain  constant  in  order  to  apply  this  rule  ? 

What  effect  is  produced  on  the  velocity  when  the  liquid  flows 
through  a long  horizontal  pipe  ? Why  ? 

What  is  the  ordinary  velocity  of  rivers  ? 

Why  should  not  a jet  of  water  escaping  vertically  rrpwards  reach 
the  level  of  the  liquid  in  the  reservoir  ? 

How  may  the  power  of  moving  water  in  a river  be  utilized? 

Describe  the  undershot  water-wheel.  Where  is  the  water  received 
in  this  wheel  ? How  are  the  float-boards  of  the  undershot-wheel  gen- 
erally placed  in  tidal  streams  ? How  are  they  placed  in  streams  which 
flow  in  only  one  direction  ? By  what  is  this  wheel  driven  ? 

Describe  the  overshot-wheel.  Where  is  the  water  received  in  this 
wheel  ? How  are  the  buckets  arranged  ? By  what  is  this  wheel 
driven  ? To  what  kind  of  streams  is  it  applicable  ? 

Describe  the  breast-wheel.  Where  is  the  water  received  in  this 
wheel?  How  are  the  buckets  arranged? 

Describe  the  turbine.  How  is  this  wheel  driven  ? Why  is  the  tur- 
bine a very  economical  form  of  water-wheel  ? 

11* 


CHAPTER  III. 

PNEUMATICS, 

145.  Properties  of  Gases.  — Gases  are  bodies  in 
which  the  force  of  molecular  repulsion  is  greater  than 
that  of  molecular  attraction.  The  molecules  are  con- 
stantly endeavoring  to  get  further  and  further  apart,  or 
in  other  words,  the  gas  constantly  exhibits  a tendency 
to  increase  in  volume,  or  to  expand.  This  tendency  to 
expand  is  called  tension. 

The  molecules  of  gases  possess  far  greater  freedom 
of  motion  than  do  those  of  liquids.  All  the  general 
properties  that  are  characteristic  of  liquids,  belong 
equally  to  gases  ; for  example  : 

1st.  Gases  transmit  'pressure  equally  in  all  direc- 
tions. 

2d.  The  downicard,  upward.,  and  lateral  pressures  at 
any  point  are  equal. 

3d.  Bodies  iveiyhed  in  air,  or  other  gas,  lose  as  much 
weight  as  the  weight  of  the  air,  or  other  gas,  they  dis- 
qolace. 

'We  shall  take  air  as  the  type  of  a gas,  since  the 
general  properties  it  possesses  belong  equally  to  other 
gases. 

146.  Expansibility  of  Gases.  — If  the  molecules 
of  a gas  are  constantly  endeavoring  to  get  further  and 

126 


PNE  EM  A TICS. 


127 


further  apart,  the  volume  of  the  gas  should  be  con- 
stantly increasing.  In  the  case  of  air,  and  other  gases 
on  the  earth’s  surface,  this  tendency  to  expand  is  held 
I in  check  by  gravity,  which  produces 
I the  pressure  of  the  atmosphere.  This 
may  be  jiroved  by  the  following  ex- 
periment. A bladder.  Fig.  61,  partly 
filled  with  air  or  other  gas,  is  tied 
: at  the  neck,  and  placed  under  a glass 
bell  connected  with  an  air-pump.  As 
the  air  is  removed  from  the  bell,  the 
gas  in  the  bladder,  being  relieved 
from  the  pressure  of  the  air  on  it,  at  Fig.  6i.— Expansion  of 
once  expands  and  fills  the  bladder,  m Vaouons  Space. 
If,  noAv,  the  air  be  allowed  to  again  enter  the  bell,  the 
gas  in  the  bladder  at  once  assumes  its  former  volume. 

147.  The  Atmosphere  is  the  name  given  to  the 
mass  of  air  which  surrounds  the  earth.  The  atmos- 
phere consists  of  a mechanical  mixture  of  two  gases, 
oxygen  and  nitrogen,  in  the  proportion,  by  volume,  of 
about  one  part  of  oxygen  to  four  of  nitrogen.  It  con- 
tains, also,  a small  fixed  quantity  of  carbonic  acid  and 
a variable  quantity  of  the  vapor  of  water. 

The  atmosphere  is  kept  in  its  place  by  the  force  of 
gravity.  We  do  not  know  the  exact  depth  of  this 
vast  ocean  of  air,  but  it  has  been  differently  estimated' 
at  from  50  to  200  miles.  These  different  estimates  are 
due  to  the  fact  that  air  becomes  less  and  less  dense,  so 
that  its  upper  surface  is  undefined. 

148.  Diffusion  of  Gases.  — If  mercury,  water,  and 
oil  be  poured  in  a tall  jar,  no  matter  how  thoroughly 
they  may  be  shaken  together,  they  will,  if  allowed  to 
stand,  soon  separate  into  three  distinct  layers,  accord- 


128 


NATURAL  PHILOSOPHY. 


ing  to  tlieir  weight.  The  mercury  being  the  heaviest, 
will  fall  to  the  bottom  of  the  jar,  the  oil  will  float  on 
top,  and  the  water  will  be  between  the  two.  This, 
however,  is  not  the  case  with  gases,  which  will  mix  or 
diffuse.  If,  for  example,  a light  gas  be  placed  in  the 
upper  part  of  a vessel,  and  a heavy  one  in  the  lower 
part,  they  will  not  remain  separate,  even  if  the  vessel 
be  kept  quite  still.  The  heavy  gas  will  rise  and  the 
light  gas  will  fall,  until  they  have  become  evenly 
mixed  or  diffused. 

As  we  have  already  seen,  many  liquids  possess,  like 
gases,  the  power  of  diffusing  through  each  other,  not- 
withstanding their  differences  of  density. 

The  property  of  diffusion  is  of  great  value  in  our  atmosphere,  which, 
as  we  have  seen,  is  composed  mainly  of  a mixture  of  three  gaseous  sub- 
stances, viz.,  oxygen,  nitrogen,  and  carbonic  acid.  Were  these  to  settle 
in  separate  layers  according  to  their  weight,  the  life  which  now  exists 
on  the  earth  would  either  entirely  cea,se,  or  be  greatly  modified.  By 
the  property  of  diffusion,  we  find  as  large  a proportion  of  the  heavier  j 
gases  in  the  air  over  the  tops  of  high  mountains  as  in  the  air  near  j 
the  earth.  I 

i 

149.  Atmospheric  Pressure. — Since  we  live  at  tbe  j 
bottom  of  the  atmosphere,  we  must,  like  everything  | 
else  at  the  earth’s  surface,  sustain  a pressure  arising  ; 
from  the  weight  of  the  air  above  us.  Gases,  however,  } 
like  liquids,  transmit  pressure  equally  in  all  directions,  | 
and  these  opposite  presstires  so  neutralize  each  other 
that  we  do  not  feel  the  pressure  which  the  air  exerts 
on  us.  If,  however,  the  pressure  should  be  removed 
from  one  side,  the  pressure  on  the  opposite  side  would 
at  once  be  felt.  The  air  presses  so  equally  on  all  sides 
of  bodies,  that  it  was  long  before  the  atmospheric 
pressure  was  discovered.  The  discovery  was  made  by 
Torricelli. 


PNEUMATICS. 


129 


i 


f 


I 

i! 

i 

1 

i' 


i 

1 

1 


150.  Torricelli’s  Experiment.  — Torricelli  made 
this  famous  discovery  by  means  of  the  following  ex- 
periment. He  took  a glass  tube  about 
33  inches  long,  closed  at  one  end, 
and  filled  with  mercury.  Placing  a 
finger  over  the  open  end,  as  in  Fig.  62, 
he  inverted  the  tube,  and  dipped  the 
open  end  below  the  surface  of  mercury 
in  a cup  or  other  vessel.  When  now 
the  finger  was  taken  away,  a column 
of  mercury  about  30  inches  high  re- 
mained in  the  tube,  being  kept  in  it 
by  the  pressure  of  the  air  on  the 
mercury  in  the  cup. 

The  atmospliere.  by  its  weight,  exerts  a downward  pressure  on  the 
surface  of  the  mercury  in  the  cup ; but  the  upward  pressure  against 
the  open  mouth  of  the  tube  is  equal  to  the  downward  pressure;  there- 
fore the  pressure  which  keeps  the  mercury  in  the  tube  is  equal  to  the 
weight  of  a,  column  of  air  of  the  same  thickness  as  the  column  of  mer- 
cury in  the  tube,  and  extending  upwards  from  the  level  of  the  mercury  in 
the  cup  to  the  up>per  limit  of  the  atmosphere. 

At  the  level  of  the  sea,  the  pressure  of  the  atmos- 
phere will  sustain  a column  of  mercury  30  inches  in 
height  above  the  level  of  the  mercury  in  the  cup.  If 
the  area  of  the  open  end  of  the  tube  be  one  square 
inch,  then  this  column  of  mercury  will  weigh  about 
fifteen  pounds.  Therefore.^  the  pressure  of  the  air  at 
the  level  of  the  sea  is  about  equal  to  15  lbs.  to  the  square 
inch. 


Fig.  62.  — The  Barom- 


151.  The  Barometer. — Torricelli’s  tube  forms  an 
instrument  called  a barometer,  by  means  of  which  we 
can  tell  the  variations  that  occur  in  the  pressure  of  the 
atmosphere.  As  the  pressure  increases,  the  mercury 
rises  in  the  barometer,  and  as  it  decreases,  it  falls.  As 


130 


NATURAL  PHILOSOPHY. 


a rule,  the  rise  of  the  mercury  in  the  barometer  at  any 
place  is  followed  by  clear  weather,  and  its  fall,  by  foul  i | 
weather.  The  barometer  is  used,  especially  at  sea,  to  J 
observe  approaching  changes  in  the  weather.  Its  use,  I 
however,  for  this  purpose  requires  considerable  expe-  i | 
rience.  J 

Since  it  is  only  the  air  above  the  mercury  in  the  cup  H 
that  keeps  the  mercury  in  the  tube,  it  follows  that  if  || 
we  carry  a barometer  up  a high  mountain,  the  height  i 
of  the  mercury  in  the  tube  will  decrease  as  we  ascend.  ■ I 
The  barometer,  therefore,  can  be  used  to  measure  the  \ 
height  of  mountains  or  other  elevations. 

152.  Accuracy  of  a Barometer. — The  space  above  l- 
the  mercury  in  the  top  of  the  tube  should  be  a com-  | 
plete  vacuum.^  that  is,  should  contain  no  air  or  other  I 
matter  except  the  unavoidable  vapor  of  mercur}'.  If  j 
air  w'ere  present  in  this  space,  it  would  prevent  the  ^ 
mercury  from  rising  as  high  as  it  should.  To  test  the  j 
completeness  of  the  vacuum,  the  tube  may  be  gently  1 
inclined,  when,  if  the  vacuum  is  complete,  a sharp  me-  j 
tallic  click  will  be  heard  as  the  mercury  fills  the  tube. 
The  mercury  also  should  be  pure,  since  otherwise  its  : 
density  would  be  changed,  which  would  of  course  aftect  i 
the  height  of  the  column. 

Barometers  can  be  made  with  other  liquids  than  mercury ; the 
height  of  the  column  will  then  depend  on  the  specific  gravity  of  the  ' 
liquid.  Thus,  if  water  were  used,  the  height  would  be  about  34  feet, 
for  since  mercury  is  13.5  heavier  than  water,  the  height  would  be  13.5  u 
X 30,  or  405  inches,  or  about  34  feet,  were  it  not  for  the  fact  that  the  .| 
vapor  of  water  formed  in  the  tube  would  somewhat  decrease  the  height.  t 

153.  Pressures  Expressed  in  Atmospheres. — We  \ 

find  it  very  convenient  to  express  the  pressure  exerted 
by  a column  of  liquid  or  gas  in  what  are  called  atmos- 
pheres of  pressure.  Thus,  if  the  pressure  is  equal  to 


PNE  UMA  TICS. 


131 


15  lbs.  to  tlie  square  inch,  we  call  it  a pressure  of  one 
atmosphere ; if  it  is  equal  to  a pressure  of  60  lbs.  to 
the  square  inch,  it  is  a pressure  of  four  atmospheres. 

154.  The  Air-Pump. — In  order  to  make  manifest 
i the  pressure  which  the  atmosphere  exerts  on  any 
object,  it  is  necessary  to  remove  the  pressure  of  the 
' air  from  one  side  of  the  object.  This  is  most  con- 
veniently done  by  means  of  the  air-pump. 

Fig.  63  represents  one  form  of  this  instrument.  An 
air-tight  piston,  P,  moves  in  the  cylinder,  C.  Open- 
ings are  provided  at  a and  c,  in  the  bottom  and  top  of 
the  cylinder,  and  at  5, 

: in  the  piston.  These 
openings  are  alternately 
I shut  and  opened  by  con- 
trivances called  valves, 
which  act  like  doors. 

In  the  air-pump  these 
I valves  all  open  up- 
I wards.  The  cylinder  is 
connected  by  a tube,  e, 
with  a flat  plate,  J/,  on 
I which  is  placed  a glass 
I vessel,  P,  called  a receiver.  By  successive  movements 
I of  the  piston,  the  air  in  the  receiver,  P,  is  gradually 
removed,  when  the  receiver  is  said  to  be  exhausted. 

The  way  in  which  the  air  is  removed  is  as  follows : 
i When  the  piston  is  raised  from  the  lower  part  of  the 
cylinder,  a vacuum  is  left  below  it,  into  which  some 
; of  the  air  from  the  receiver,  P,  at  once  passes,  lifting 
i by  its  tension  the  valve  a.  When  the  piston  is  pushed 
i down,  the  air  in  C is  compressed,  the  valve  a shut,  and 
I the  valve  h opened,  and  the  air  below  the  piston  is 

i 

! 


132 


NATURAL  PHILOSOPHY. 


now  transferred  above  it.  As  tlie  piston  is  again  1 
raised,  some  more  air  passes  from  R to  the  cylinder, 
and  the  air  above  the  piston  is  forced  out  of  the  cyl- 
inder through  the  valve  c. 

155.  Illustrations  of  Atmospheric  Pressure. — The 

receiver,  A,  can  easily  be  lifted  from  the  plate  of  the 
air-pump,  provided  the  air  is  not  removed  from  the 
inside.  As  soon,  however,  as  the  receiver  is  exhausted, 
the  pressure  of  the  air  on  the  outside  fixes  it  so  firmly 
to  the  plate,  that,  if  the  receiver  is  moderately  large, 
it  will  be  almost  impossible  for  a person  to  remove  it 
from  the  plate  until  air  is  again  allowed  to  enter. 

Two  hollow  hemispheres  of  brass  Avith  smooth  plane 
edges,  if  simply  pressed  together  so  as  to  form  a hol- 
low sphere,  and  then  connected  with  the  air-pump  so 
that  the  air  may  be  removed  from  the  inside,  are  j 
held  together  so  firmly  by  the  pressure  of  the  air  on 
the  outside  that  it  is  very  difficult  to  pull  them  apart.  1 

When  a receiver  with  an  open  top,  over  which  a 
piece  of  bladder  has  been  tightly  stretched,  is  placed  on  ' 
the  plate  of  an  air-pump  and  exhausted,  the  pressure  I 
of  the  air  on  the  bladder  bursts  it  with  a loud  report. 

156.  Simple  Experiments  in  Atmospheric  Press-  i 
ure. — The  following  experiments  can  be  shown  without 
the  use  of  an  air-pump. 

ist  Experiment. — Place  the  open  end  of  a hollow  key  to  the  mouth,  ' | 
and  vigorously  sucking  out  the  air,  quickly  press  it  against  the  lip,  ' 1 
and  it  will  be  held  there  by  the  pressure  of  the  air.  i 

2d  Experiment.  — Select  a small  wine-glass  with  a smooth  edge.  Jj 
Place  some  small  pieces  of  paper  loosely  in  the  glass,  and  set  fire  to  I 
them.  As  soon  as  they  are  nearly  consumed,  quickly  press  the  glass  I 
to  the  hand,  and  it  will  then  be  pressed  against  it  with  considerable  j 
force.  The  heat  expands  the  air  and  drives  part  of  it  out  of  the  I ' 
glass,  which  is  then  held  against  the  hand  by  the  outside  atmospheric  i j 
pressure.  J 


PNE  UMA  TICS. 


133 


Fig,  64,  — An  Experiment 
in  Atmospheric  Pressure. 


3d  Experiment. — Try  the  same  with  a glass  preserving  jar,  only 
place  the  open  end  of  the  jar  below  the  surface  of  water  in  a soup 
plate.  As  the  jar  cools,  the  water  will  rise  and  partly  fill  the  jar. 

Caution. — It  is  not  necessary  in  either  of  these  experiments  to  have 
the  glass  very  hot.  Select  thin  glass,  which  is 
less  apt  to  crack  when  suddenly  heated  or 
cooled. 

4th  Experiment — Fill  a smooth-edged  tum- 
bler with  water,  place  a piece  of  stout  paper 
over  the  top,  and  pressing  the  palm  of  the 
hand  against  the  paper,  slowly  invert  the 
tumbler.  The  hand  may  now  be  removed 
from  the  p.aper,  and  the  water  will  not  run 
out,  since  the  pressure  of  the  air  keeps  it  in. 

5th  Experiment. — Select  an  empty  tomato 
or  other  tin  can,  from  the  top  of  which  the  small,  round  piece  of  tin 
only  has  been  removed,  leaving  an  opening  about  two  inches  in  diameter. 
Tie  a piece  of  mosquito  netting  firmly  around  this  end  of  the  can, 
stretching  it  smoothly  over  the  top  with  the  fingers.  Fill  the  can  with 
water  by  pouring  it  through  the 
meshes  of  the  netting.  Place  a 
piece  of  smooth,  stiff  paper  over 
the  open  end,  and  invert  as  in  the 
previous  experiment.  The  paper 
will  then  be  held  against  the  can 
by  the  pressure  of  the  air.  Now 
holding  the  can  as  shown  in  Fig. 

65,  cautiously  slide  the  paper  from 
the  netting,  a7id  the  water  will  still 
remain  in  the  can,  although  the 
open  end  is  only  protected  by  the 
mosquito  netting. 

6th  Experiment.  — Prepare  a 
can  as  described  in  the  5th  exper- 
iment, but  in  addition  punch  a hole  with  a nail  in  the  bottom  of  the 
can.  Holding  a finger  firmly  against  this  hole,  fill,  invert,  and  re- 
move the  paper  as  before,  and  the  water  will  not  run  out : now  remove 
the  finger  momentarily  from  the  hole,  as  shown  in  Fig.  66.  The  press- 
ure of  the  air  is  then  exerted  downwards  on  the  water  as  well  as 
upwards,  and  the  water  flows  out  by  its  own  weight.  Eeplace  the 
finger,  and  the  flow  after  a moment  ceases,  since  the  pressure  of  the  at- 
mosphere is  greater  than  the  weight  of  the  water.  This  curious  experi- 
12 


Fig.  65.—  An  Experiment  in  Atmospheric 
Pressure. 


134 


NA  T UR  A L PHIL  0 SO  PHY. 


merit  proves,  1st.  Atmospheric  pres.sure  ; 2d.  Considerable  adhesion  of  i 
the  water  to  the  netting,  and,  3d.  Considerable  cohesion  of  the  molecules 
of  the  water.  To  succeed  with  it,  read  carefully  the  following 

Caution. — The  netting  must  be  free  from  grease,  and  the  open  end  i 
of  the  can  smooth  ; above  all,  the  can  must  in  all  cases  be  held  with  its  t 

open  end  as  nearly  horizontal  as  possible.  If  you  do  not  at  first  succeed  ^ 

with  this  or  any  other  experiment,  try  again 
until  you  are  successful.  The  ability  to  suc- 
cessfully experiment  can  only  be  acquired  by 
practice. 

The  common  leather  sucker  depends  for  its  I ■ 
operation  on  the  pressure  of  the  air.  j 

157.  Buoyancy  of  Air. — A body  j 
wlieu  weiglied  iu  air  loses  as  muck  , 
weight  as  the  weight  of  the  air  it  dis-  i 
places.  For  ordinary  purposes  this  '■ 
lo.ss  of  Aveight  may  be  disregarded;  ^ 
it  becomes  more  considerable  in  pro- 
portion as  the  bulk  of  the  thing  weighed  exceeds  that 
of  the  Aveights  used  to  balance  it. 

Suppose,  for  example,  a pound  of  feathers  be  balanced  in  air  by  a 
pound  of  lead  ; then,  since  the  bulk  of  the  feathers  is  greater  than  that 
of  the  lead,  the  buoyancy  of  the  air  must  decrease  their  weight  more 
than  the  weight  of  the  lead,  ajid  therefore  more  than  one  pound  of  \ ' 
feathers  must  be  taken  to  balance  one  pound  of  lead.  | < 

158.  Balloons. — solid  lighter  than  AA'ater,  im- 
mersed beloAV  the  surface  of  Avater,  Avill,  unless  held  in 
place,  rise  through  the  Abater  until  it  reaches  the  sur- 
face, Avhere  it  Avill  float.  Balloons  rise  through  the  air 
for  the  same  reason,  for,  being  filled  AA’ith  some  light  gas  '' 
or  heated  air,  their  AA'eight,  Avhich  tends  to  pull  them 
doAA'u,  is  less  than  the  buoyant  force  of  the  displaced 
air  which  tends  to  push  them  up.  "When  these  tAA'o 
forces  are  exactly  equal,  the  balloon  AA'ill  neither  rise 
nor  fall.  The  ascensional  or  liftiii'j  power  of  a balloon 
can  therefore  be  found  by  subtracting  the  AA'eight  of  } ‘ 


Fig.  66.  — An  Experiment 
in  Atmospheric  Pressure. 


PNE  UMA  TICS. 


135 


the  balloon,  enclosed  gas,  and  car  from  the  weight  of 
an  equal  bulk  of  air.  100  cubic  inches  of  air  at  the 
sea-level  weigh  about  31  grains. 

159.  Effect  of  Pressure  on  the  Volume  of  a Gas. 

— Gases,  as  we  have  seen,  are  the  most  compressible 
forms  of  matter.  As  the  pressure  on  any  bulk  of  gas 
is  increased,  its  volume  is  diminished,  and,  conversely, 
as  the  pressure  is  decreased,  the  tension  of  the  gas 
causes  its  volume  to  increase.  The  law  according  to 
which  these  changes  occur  was  discovered  by  Mariotte 
and  Boyle,  and  may  be  expressed  as  follows : 

At  the  same  temperature.^  the  volume  occupied  hy  any 
hulk  of  air  is  inversely  proportional  to  the  pressure  it 
supports.  This  law  is  very  nearly  true  for  all  gases. 

Suppose,  for  example,  a certain  quantity  of  air  occupies,  at  the  ordi- 
nary pressure  of  the  atmosphere,  the  volume  of  one  quart ; then,  if  the 
pressure  on  this  mass  of  air  be  increased  to  two  atmospheres,  its  volume 
will  be  reduced  to  one-half  a quart;  if  the  pressure  be  increased  to 
three  atmospheres,  its  volume  will  be  reduced  to  one-third  of  a quart ; 
if  to  ten  or  one  hundred  atmospheres,  to  -j-'j-  or  of  a quart.  Again, 
if  the  pressure  of  one  atmosphere  on  the  quart  be  reduced  to  one- half 
an  atmosphere,  the  tension  of  the  air  will  cause  it  to  expand  to  two 
quarts  ; if  it  be  reduced  to  of  an  atmosphere,  it  will  expand  to 
one  hundred  quarts.  The  ability  of  a gas  to  expand  on  the  relief  of 
pressure  appears  to  be  almost  unlimited. 

160.  Effect  of  Pressure  on  the  Specific  Gravity 
or  Density. — Since  the  mass  of  a gas  remains  the 
same,  however  its  volume  may  be  changed  by  press- 
ure, the  density  or  specific  gravity  must  increase  as  the 
pressure  increases,  or,  in  other  words,  the  density  must 
he  proportional  to  the  pressure. 

Since  the  lower  layers  of  the  atmosphere  sustain  the  weight  of  the 
upper  layers,  the  density  of  the  air  near  the  sea-level  is  greater  than 
that  of  the  air  over  the  top  of  a mountain.  Assuming  the  height  of  the 
atmosphere  to  be  from  50  to  200  miles  above  the  sea-level,  by  far 
the  greater  mass  of  the  air  lies  within  a few  miles  of  the  general  surface. 


136 


NATURAL  PniLOSOPET. 


Fig.  67.— The  Siphon. 


161.  Machines  Depending  for  their  Action  on 
Atmospheric  Pressure.  — In  the  following  machines, 
advantage  is  taken  of  the  pressure  of  the  atmosphere. 

1.  The  Siphon.  — The  siphon  consists  of  a tube 
bent  as  shown  in  Fig.  67.  When  the 
shorter  arm  is  placed  below  a water- 
surface,  and  the  tube  exhausted  by 
the  mouth  applied  at  6,  the  pressure 
of  the  air  on  the  water  in  m causes 
the  water  to  rise  through  the  height, 
m 71,  and  flow  out  of  the  open  end  of 
the  tube,  h.  The  greater  the  differ- 
ence of  level,  a h,  between  the  water- 
surface,  m,  and  the  open  end,  h,  the 
greater  the  velocity  with  which  the 
water  will  escape. 

2.  The  Suction  Water-Pump. — -In  the  common  suc- 
tion-pump, Fig.  68,  the  w'ater  is  forced  up  out  of  the  well 
into  the  body  of  the  pump  by  the  pressure  of  the  at- 
mosphere. In  its  simplest  form,  this 
pump  is  essentially  the  same  as  the 
common  air-pump.  The  valves  open 
upwards  ; one  or  more  valves,  h h,  are 
placed  in  the  piston,  and  one,  a,  at  the 
lower  end  of  the  cylinder,  or  pump- 
barrel.  As  the  piston  is  raised,  a 
vacuum  is  left  below  it  in  the  pump- 
barrel,  into  which  rushes  the  air 
from  the  pipe  dipping  down  into  the 
well,  ir.  As  the  air  is  thus  sucked 

Fig.  68.  — The  Suction-  out  of  the  pipe,  the  pressure  of  the 
Pump.  water  in  IF  forces  it  up 

through  the  valve,  a.  As  the  piston  descends,  the 
valve  a closes  and  the  valves  h h open,  and  the 


SYLLABUS. 


137 


water  passes  above  tne  piston.  When  the  piston  is 
again  raised,  the  iVater  escapes  at  the  mouth  of  the 
pump  at  D.  In  the  figure,  the  valves  are  represented 
in  the  position  they  w'ould  occupy  in  the  up-stroke  of 
the  piston. 

3.  The  Force-Pump.  — In  the  force-pump.  Fig.  69, 
there  is  no  valve  in  the  piston,  P.  A pipe,  T.,  enters 
the  side  of  the  cylinder  near  the  bottom.  A valve,  a, 
which  opens  outwards,  is  placed  where  this  tube  enters 
the  cylinder.  The  valve  h is  placed,  as 
before,  at  the  lower  end  of  the  barrel. 

On  the  downward  stroke  of  the  piston, 
the  water,  instead  of  passing  above  the 
piston,  is  forced  through  the  valve  a 
up  through  the  pipe,  T.  In  the  figure 
the  valves  are  represented  in  the  posi- 
tions they  would  occupy  during  the 
up-stroke  of  the  piston. 

The  height,  m n,  through  which  the  water  is 
raised  by  the  pressure  of  the  air  in  the  siphon,  the 
suction-,  or  the  lifting-pump,  can  never  exceed  34  69.— The  loroe- 

feet,  since,  as  we  have  seen,  a column  of  water  Pnmp. 

of  this  height  exerts  a pressure  equal  to  that  of  the  atmosphere.  In 
practice,  pumps  seldom  raise  water  higher  than  28  feet  from  the  level 
of  the  well  to  that  of  the  lower  valve. 

Syllabus. 

Gases,  like  liquids,  1st.  Transmit  pressure  as  well  in  one  direction  as 
in  another;  2d.  Exert  equal  upward,  downward,  and  lateral  pressures 
at  the  same  part  of  their  mass,  and  3d.  Exert  a buoyant  force  on  any 
body  immersed  in  them. 

The  atmosphere  consists  of  a mixture  of  four  parts  by  volume  of 
nitrogen  and  one  part  of  oxygen.  It  also  contains  carbonic  acid  and 
the  vapor  of  water.  The  height  of  the  atmosphere  has  been  variously  es- 
timated at  from  50  to  200  miles ; its  upper  surface,  however,  is  undefined. 


138 


NATURAL  PniLOSOPIIT. 


The  atmosphere  has  weight,  and  therefore  exerts  a pressure  on  all 
things  on  the  earth’s  surface.  The  reason  that  this  pressure  is  not 
generally  felt,  is  because  it  is  exerted  equally  in  all  directions. 

The  fact  that  the  atmosphere  exerts  a pressure  on  the  earth,  was  dis- 
covered by  Torricelli  by  means  of  an  instrument  called  the  barometer. 

The  mercury  is  sustained  in  the  barometer  tube  by  means  of  the 
pressure  which  the  air  exerts  on  the  surface  of  the  mercurj'  in  the  cup 
in  which  the  barometer  tube  dips.  At  the  level  of  the  sea,  the  height 
of  the  barometric  column  is  about  30  inches.  This  is  equal  to  a pressure 
of  15  lbs.  on  each  square  inch  of  surface. 

The  barometer  is  used,  1st.  To  indicate  approaching  changes  in  the 
weather,  and  2d.  To  measure  the  height  of  mountains  or  other  eleva- 
tions. 

The  accuracy  of  the  barometer  depends  on,  1st.  The  completeness 
of  the  vacuum  in  the  upper  part  of  the  tube,  and  2d.  On  the  purity 
of  the  mercury. 

By  a pressure  of  one  atmosphere,  we  mean  a pressure  of  15  lbs.  on 
each  square  inch  of  surface. 

When  the  pressure  of  the  air  is  removed  from  any  one  side  of  a 
body,  its  pressure  on  tbe  other  side  at  once  becomes  manifest. 

Bodies  weighed  in  air  lose  an  amount  of  weight  equal  to  tbe  weight 
of  the  air  they  displace  ; therefore,  a pound  of  feathers  balanced  in  air 
by  a pound  of  lead  is  actually  heavier  than  the  lead. 

Balloons  rise  through  the  air  because  the  weight  of  the  air  they  dis- 
place is  greater  than  their  own  weight. 

At  the  same  temperature,  the  volume  occupied  by  any  bulk  of  air  i.? 
inversely  proportional  to  tbe  pressure  it  sustains.  Its  density  or  spe- 
cific gravity  is  directly  proportional  to  the  pressure. 

The  lower  layers  of  the  atmosphere  are  denser  than  the  upper 
layers,  because  tbe  lower  layers  have  to  sustain  the  wmight  of  all  the 
air  above  them. 

A liquid  is  forced  up  through  the  short  arm  of  a siphon,  or  from  a 
well  into  the  body  of  a pump,  by  the  pressure  of  the  air. 

In  the  suction-pump  and  in  the  force-pump,  there  is  a valve  placed 
in  the  lower  part  of  the  cylinder,  directly  over  the  pipe  leading  down 
into  the  well.  In  the  suction-pump,  a valve  or  valves  are  placed  in 
the  piston.  In  the  force-pump  the  piston  is  solid,  and  a valve  is  placed 
in  the  side  of  the  cylinder,  near  the  bottom,  directly  over  the  point 
where  a pipe  enters  the  cylinder. 

The  greatest  distance  that  water  can  be  raised  from  a well  to  the 
barrel  of  the  pump  is  3-1  feet.  It  is  seldom  raised  more  than  2S  feet. 


QUESTIONS  FOR  REVIEW. 


139 


Questions  for  Review. 

Name  any  properties  that  are  possessed  in  common  by  both  gases 
and  liquids. 

Of  what  gaseous  substances  does  the  atmosphere  consist  ? 

What  is  known  of  the  upper  limit  of  the  atmosphere  ? 

Why  do  we  not  feel  the  pressure  which  the  air  exerts  upon  us  ? 
How  did  Torricelli  prove  that  the  atmosphere  exerts  a pressure  on 
everything  it  touches  ? What  is  the  atmospheric  pressure  in  pounds 
per  square  inch  ? 

Describe  the  barometer.  For  what  purposes  is  the  barometer  used? 
Upon  what  does  the  accuracy  of  a barometer  depend?  How  high 
would  the  column  of  a water  barometer  be  ? 

What  is  meant  by  a pressure-  of  one  atmosphere  ? Describe  the 
operation  of  the  air-pump. 

Describe  in  full  any  simple  experiments  by  which  the  pressure  of  the 
air  can  be  demonstrated.  Describe  an  experiment  with  a tomato  can 
and  a piece  of  mosquito  netting. 

What  effect  has  the  buoyancy  of  air  on  the  weight  of  bodies? 
What  causes  balloons  to  rise  through  the  air?  How  may  the  ascen- 
sional power  of  a balloon  be  calculated  ? 

What  effect  has  an  increase  of  pressure  on  the  volume  of  a gas  ? 
What  effect  has  a decrease  of  pressure?  What  effect  has  either  an 
increase  or  a decrease  of  pressure  on  the  density  of  a gas  ? State  the 
law  of  Mariotte  and  Boyle. 

Why  are  the  lower  layers  of  the  atmosphere  denser  than  the  upper 
layers  ? 

Describe  the  construction  and  operation  of  the  siphon. 

How  many  valves  are  there  in  a suction-pump  ? Where  are  they 
placed  ? How  do  they  open  ? In  what  respects  are  the  suction-pump 
for  water  and  the  air-pump  alike? 

How  many  valves  are  there  in  a force-pump?  Where  are  they 
placed  ? How  does  this  pump  differ  from  a suction-pump  ? 

Why  cannot  water  be  lifted  by  the  pressure  of  the  air  higher  than 
34  feet  from  the  surface  of  water  in  a well  to  the  barrel  of  a pump  ? 


1! 


Part  III.  | 

Sound  and  Heat.  I 

CHAPTER  I.  i 

THE  CAUSE,  TRANSMISSION,  REFLECTION,  ' 

AND  REFRACTION  OF  SOUND. 

162.  Sound  Defined. — Sound  is  caused  by  a vibra-  * 

tory  or  wave- like  motion  of  the  air  or  other  medium, 
the  effect  of  which  is  transmitted  by  the  ear  to  the 
brain.  ’ 

W e use  the  word  sound  in  two  distinct  senses,  viz. : 

1st.  As  the  impression  which  is  produced  on  the 
brain,  and 

2d.  As  the  thing  which  causes  the  impression,  viz., 
as  the  vibrations  of  the  air  or  other  medium.  It  is 
evident,  however,  that  the  sound  we  hear  is  the  im- 
pression, while  the  thing  that  causes  the  impression 
is  the  vibrations  of  the  air. 

163.  Nature  of  Wave-Motion.  — Since  sound  is 

caused  by  a vibratory  or  wave-motion  of  the  air,  it  is 
necessary,  before  studjdng  the  phenomena  of  sound,  to 
obtain  clear  ideas  of  the  nature  of  wave-motion.  » 

When  a wave-motion  is  started  in  a body  of  deep  * 

TiO  I 


SOUND. 


141 


water,  it  appears  as  if  the  whole  mass  of  water  at  the 
surface  were  moving  in  the  direction  in  which  the 
wave  is  advancing.  If,  however,  we  observe  any 
light  body  floating  on  the  water,  we  wdll  notice  that 
it  merely  rises  and  falls,  and  does  not  advance  with  the 
wave.  The  water,  therefore,  cannot  be  moving  bodily 
forward.  Similar  wave  movements  are  seen  in  the 
shaking  of  carpets  or  cords. 

If  a cord,  A i?.  Fig.  70,  fixed  at  A,  be  smartly  shaken 
bv  the  hand  at  B,  a wave-motion,  as  shown  by  the  con- 


Fig.  70.— Waves  in  a String. 

string  in  a direction  from  B to  A.  On  reaching  J.,  it 
will  be  reflected,  and  will  move  back  towards  the  hand 
along  the  dotted,  curved  line.  These  motions  give  the 
particles  of  the  string  the  appearance  of  moving  alter- 
nately from  B to  A and  from  A to  B.  This,  of  course, 
is  not  the  case,  the  particles  merely  rising  and  falling; 
being  sometimes  above  the  straight  dotted  line,  as  at 
B a E,  and  sometimes  below  it,  as  at  ^ i D. 

In  wave-motion.,  the  particles  move  alternately  hack- 
wards  and  forwards  through  comparatively  short  dis- 
tances, while  the  waves  themselves  may  move  in  only 
one  direction  through  considerable  distances. 

Thus,  let  the  dots  from  D to  B,  Fig.  71,  represent  a 
row  of  particles 
which,  when  at 
rest,  will  have 
the  position  as 
shown  in  the 
straight  line  B Fig.  71.—  Motion  of  the  Particles. 

B.  If,  now,  a wave  moves  along  the  string  from  B to 


Lious  curved  line,  will  move  along  the 


142 


NATURAL  PHILOSOPHY. 


2),  the  particles  do-  not  move  from  B io  D \ they  sim- 
ply move  at  one  moment  below  the  straight  line  D B, 
and  at  the  next  moment  above  it. 

164.  Definitions.  — The  alternate  motion  to  and 
fro,  from  one  side  of  the  position  of  equilibrium  to 
the  other,  is  called  a vibration.^  undulation.^  or  wave. 
Thus,  in  Fig.  70,  the  portion  B a E above  the  straight 
line  A B.,  together  with  the  portion  E h D below  this 
line,  form  one  complete  wave  movement. 

The  length  of  the  wave  is  the  distance,  B 21,  measured 
along  the  straight  line,  or  is  the  shortest  distance  be- 
tween any  two  sets  of  particles  that  are  moving  at  the 
same  time  in  the  same  direction. 

The  amplitude  of  the  wave  is  the  line  6 2 or  c e,  Fig. 
71,  Avhich  marks  the  greatest  distance  that  any  particle 
has  moved  out  of  its  position  of  equilibrium. 

The  wave  period.,  or  the  time  of  vibration,  is  the 
time  required  for  any  particle  to  make  one  complete 
movement  from  one  side  of  its  position  of  equilibrium 
to  the  other.  In  Avaves  of  the  same  length,  the  wave 
period,  or  time  of  vibration,  is  the  same,  AvhateAmr  may 
be  the  amplitude  of  the  Avave.  If,  therefore,  a tightly 
stretched  cord,  such  as  a string  in  a harp  or  Auolin,  be 
pushed  by  the  hand,  it  will,  before  coming  to  rest, 
move  to  and  fro  betAveen  its  position  of  rest  through  a 
gradually  smaller  and  smaller  amplitude;  but  the  time 
of  its  vibration  aaTII  be  the  same  near  the  end  of  its 
motion  as  near  the  beginning. 

165.  The  Cause  of  Sound.  — If  AA"e  cause  a bell  to 
sound  by  striking  it,  we  can,  by  lightly  touching  its 
sides,  feel  that,  while  it  is  sounding,  its  sides  are  shak- 
ing to  and  fro;  and  if  it  is  sounding  loudly,  Ave  can 


SOUND. 


143 


even,  see  its  sides  shaking.  When  these  shakings 
cease  the  sound  ceases,  as  we  can  prove  by  pressing 
the  hand  against  the  sides,  and  thus  stopping  the  vi- 
brations, when  the  sound  at  once  ceases.  It  can,  in  a 
like  manner,  be  shown  that  all  bodies  which  are  pro- 
ducing sound  are  vibrating. 

Sound  is  caused  hy  the  shakings  or  vibrations  of  the 
sounding  or  sonorous  body. 

A tuning  fork  consists  of  a bar  of  steel,  of  the  form 
shown  at  a 6,  Fig.  72,  and  firmly  supported  on  a hollow 
case,  c,  of  dry  wood.  When  the  fork 
is  sounded  by  rubbing  the  sides  near 
the  top  by  a rosined  bow,  the  arms, 
a'b,  move  alternately  towards  and 
from  each  other,  and,  setting  the  air 
around  them  into  Avaves,  produce  a 
musical  sound. 

Experiment. — Partially  fill  a thin  glass  goblet  with  water,  and  rub 
the  moistened  finger  lightly  against  the  edge,  so  as  to  cause  a clear 
musical  note.  The  surface  of  the  water  will  then  he  rufified  with  min- 
iature waves,  produced  hy  the  shakings  or  vibrations  of  the  sides  of 
the  goblet. 

Caution.' — To  obtain  a strong,  clear  note,  the  motion  of  the  finger 
must  be  regular. 

166.  Manner  in  which  the  Motion  is  Conveyed 
from  the  Vibrating  Body  to  the  Ear.  — The  sides  of 
the  vibrating  body  set  the  surrounding  air  into  waves, 
which  move  out  from  the  body  in  all  directions.  These 
Avaves  reaching  the  ear,  cause  us  to  have  the  impres- 
sion of  sound. 

167.  Nature  of  Sound-Waves, — When  any  sono- 
rous body,  as,  for  example,  a bell,  is  set  into  vibration, 
as  its  sides  are  moving  outwards,  they  croAvd  the  par- 
ticles of  air  immediately  in  front  of  them  into  a smaller 


144 


NATURAL  PHILOSOPHY. 


space,  and  thus  cause  a condensation  of  the  air.  But 
when  its  sides  are  moving  in  the  opposite  direction,  the 
particles  of  air  not  being  able  at  once  to  follow  them, 
get  further  apart,  and  thus  produce  a rarefaction  of  the 
air.  A complete  vibration  of  the  bell  consists  of  a mo- 
tion of  its  walls  outwards  and  inwards.  A complete 
wave-motion  of  the  air  consists  of  an  alternate  con- 
densation and  rarefaction. 

The  sound-waves  that  are  produced  in  the  air  by 
the  vibrations  of  a sonorous  body  move  outwards  from 
it  in  all  directions,  and  consist  of  alternate  condensa- 
tions and  rarefactions  of  the  air.  They  surround  the 
sonorous  body  in  the  shape  of  constantly  increasing 
spherical  shells.  The  motions  are  called  waves  of  con- 
densation and  rarefaction. 

The  motion  of  the  air  particles  in  a sound-wave  is  al- 
ternate! v backwards  and  forwards  in  the  same  direction 
as  that  of  the  line  in  which  the  wave  is  advancing.  The 
amplitude  of  a sound-wave  depends  on  the  degree  of 
rarefaction  and  condensation.  The  greater  the  num- 
ber of  particles  crowded  into  the  condensed  space,  and 
the  smaller  the  number  in  the  rarefied  space,  the  greater 
the  amplitude. 

Some  idea  of  the  nature  of  these  waves  may  be 
obtained  from  Fig.  73,  where  the  shaking  of  the  bell 
is  represented  as  causing  waves  in  the  air  around  it. 
In  these  waves,  the  particles  of  air  are  alternately 
crowded  together  and  separated  from  one  another,  as 
shown  by  the  dark  and  light  shadings. 

The  nature  of  the  motion  of  the  particles  of  air  may 
be  roughly  represented  by  placing  a number  of  glass 
marbles  in  a straight,  wooden  trough,  so  that  they  all 
touch  one  another.  If,  now,  a marble  at  one  end  of 
the  trough  be  rolled  sharply  against  the  marble  next 


S 0 UND. 


145 


it,  the  balls  will  not  move  on  together ; the  first  ball 
gives  its  motion  to  the  second,  and  then  stops ; the 


second  gives  its  motion  to  the  third,  and  then  stops ; 
and  so  on  through  all  the  balls  to  the  end,  which,  hav- 
ing no  ball  to  give  its  motion  to,  moves  on.  So  with 
the  sound-waves,  the  vibrating  body  strikes  the  parti- 
cles of  air  near  it ; these  give  their  motion  to  the  par- 
ticles of  air  beyond  them,  and  then  stop ; and  these 
particles  to  others  beyond  them,  and  so  on  until  at  last 
the  waves  reach  the  ear  and  we  hear  the  sound. 


168.  A Medium  Necessary  to  Transmit  Sound. — - 

Since  sound  is  transmitted  by  waves,  something  must 
exist  between  the  sounding  body  and 
the  ear  to  be  set  into  waves,  in  order 
that  the  sound  may  be  carried  from  one 
point  to  another.  If  a bell  be  struck 
in  a vacuum,  no  sound  will  be  heard, 
since  there  would  be  no  medium  to  be 
set  into  waves  to  carry  the  sound.  In 
Fig.  74,  a bell  is  suspended  by  a thread 
inside  a glass  globe,  Ji.  If  the  air  be  removed  from 
the  globe  by  means  of  the  air-pump,  no  sound  will 
be  heard  when  the  bell  is  struck. 

13  K 


Fig.  74.— BeU  in 
Globe. 


146 


NATURAL  PHILOSOFHY. 


Sound-waves  are  transmitted  througli  the  air  be-« 
cause  the  air  is  elastic.  Any  elastic  substance  canm 

transmit  sound.  Therefore,  gases,  liquids,  and  solids  1 
will  all  act  as  sound  media,  that  is,  will  transmit! 
sound.  I 

Experiment. — Hold  a bell  under  water,  and  ring  it.  The  sound  T 
can  be  heard  by  those  standing  near  the  water.  Here  the  water  must  } 
have  transmitted  the  motions  of  the  bell.  I 

Caution.  — Of  course  the  bell  will  sound  differently  in  water  than  I 
in  air.  1 

Experiment.  — Remove  the  bottoms  of  two  small  tomato-cans;  I 
moisten  a piece  of  bladder  or  stiff  paper  and  stretch  it  tightly  over  the  | 
open  end  of  each,  and  secure  by  tying.  When  quite  dry,  pierce  a hole  1 
in  the  middle  of  each  end  with  a large  darning-needle.  Get  a piece  f 
of  string  15  or  20  feet  long,  and  run  one  end  through  each  of  these  J 
holes  from  the  outside  of  the  can  to  the  inside.  Tie  a piece  of  stick  ( 


Fig.  75. — The  String-Telephone. 


to  each  end  of  the  string  to  prevent  its  slipping  out.  If,  now,  the 
string  is  stretched  rather  tight,  a person,  by  placing  the  ear  at  the 
opening  of  one  of  the  cans,  can  distinctly  hear  all  another  person 
whispers  into  the  open  end  of  the  other  can.  This  instrument  is  called 
the  string-telephone. 

Caution. — The  bladder  must  be  quite  tight.  A faint  whisper  at  one 
end  should  be  distinctly  heard  at  the  other  end.  t 

Experiment. — Try  the  same  experiment,  using  cig.ar-hoxes  and  wire  « 
instead  of  the  tomato-can,  string,  and  bladder.  Bore  a hole  in  the  j 


SOUND. 


147 


bottom  of  each  box  for  the  insertion  of  the  wire.  Talk  into,  or  listen 
at  the  open  end  of  the  boxes.  The  wire  may  then  be  stretched  for  a 
considerable  distance  in  a straight  line,  as  from  one  house  across  a 
street  to  another. 

Caution. — Select  for  this  purpose  boxes  with  thin,  elastic  bottoms. 

169.  Velocity  of  Sound.  — Time  is  required  for 
sound  to  travel  from  one  place  to  anotlier.  We  can 
see  a distant  man  strike  a blow  with  a hammer  some 
time  before  we  hear  the  sound.  We  see  the  lightning 
which  causes  the  thunder  before  we  hear  the  thunder. 

The  velocity  of  sound  in  air  varies  with  the  tempera- 
ture. At  the  temperature  of  freezing  water,  sound 
travels  1090  feet  in  every  second.  Sound  travels  in 
warm  air  more  rapidly  than  in  cold  air.  For  each 
degree  of  temperature  above  the  freezing-point,  or 
32°  Fah.,  the  velocity  increases  about  lyg  feet. 

In  the  same  medium,  all  ordinary  sounds  have  the 
same  velocity ; since  we  can  enjoy  the  music  of  an 
orchestra  as  well  when  we  are  at  a distance  from  it  as 
when  we  are  near,  the  different  sounds  produced  must 
all  move  with  equal  velocity,  for,  if  they  did  not,  some 
sounds  would  reach  us  sooner  than  others,  and  thus 
cause  discord. 

The  velocity  of  sound  in  toater  is  about  four  and  one- 
half  times  greater  than  the  velocity  in  air. 

The  velocity  of  sound  in  elastic  solids  is  generally 
greater  than  in  either  air  or  water.  Thus,  the  velocity 
in  cast-iron  is  about  ten  and  one- half  times  greater 
than  the  velocity  in  air. 

170.  Reflection  of  Sound. — When  nothing  inter- 
feres  with  their  progress,  sound-waves  move  in  all 
directions  in  straight  lines.  But  when  they  meet  with 
a suitable  obstacle,  they  are,  like  other  elastic  bodies. 


148 


NATURAL  PHILOSOrEY. 


reflected  or  thrown  off  from  it  at  an  equal  angle  to 
that  at  which  they  struck  it.  This  change  in  the  di- 
rection of  the  sound-waves  is  known  as  the  reflection  of 
sound. 

Tlie  smooth  and  hard  surfaces  of  elastic  bodies  are 
the  best  reflectors  of  sound-waves.  Cloth.s,  curtains, 
or  other  draperies  scarcely  reflect  the  w'aves  at  all.  A 
smooth  water  surface  forms  an  excellent  reflector  of 
sound. 

171.  Echoes. — When  an  impression  on  the  brain 
has  been  made  by  the  sound-waves  through  the  ear, 
the  sensation  continues  for  a short  time.  If,  therefore, 
two  sounds  follow  each  other  too  rapidly,  the  ear  will 
be  unable  to  distinguish  them  as  separate  sounds,  and 
the  effect  of  one  continuous  sound  will  be  produced. 
Sounds  like  those  of  the  voice,  as  in  speaking,  must 
be  about  the  fifth  of  a second  apart,  in  order  to  be 
heard  separately. 

If  we  stand  in  front  of  a sufficiently  large  and  dis- 
tant reflecting  surface,  such,  for  example,  as  a high 
wall,  and  speak  in  a loud  voice,  we  will  hear  two  sepa- 
rate sets  of  sounds,  viz.,  1st.  Those  produced  on  the 
ear  by  the  direct  sound,  and,  2d.  Those  produced 
by  the  sound-waves  which  have  been  reflected  by  the 
distant  object  back  again  to  us.  These  latter  sounds, 
which  are  produced  by  the  reflected  sound-waves,  are 
called  echoes. 

If  the  time  required  for  the  sound-waves  to  move 
towards  and  from  the  reflecting  surface  is  no  shorter 
than  one-fifth  of  a second,  the  reflected  sound  will  be 
heard  separately  from  the  direct  sound  ; if  the  time 
be  less  than  this,  the  direct  and  reflected  sounds  will 
blend  with  each  other. 


S’  0 UND. 


149 


At  the  temperature  of  60°  Fah.,  the  velocity  of  sound  is  about  1120 
feet  per  second.  During  the  fifth  of  a second,  the  waves  would  travel 
224  feet.  If,  therefore,  a person  is  standing  in  front  of  a reflecting 
surface  which  is  112  feet  distant,  and  is  speaking  at  the  rate  of  five 
syllables  per  second,  the  reflected  sound  could  go  and  return  so  as  to 
be  heard  before  the  next  syllable  would  be  pronounced,  and  thus  both 
the  direct  and  reflected  sound  could  be  heard.  Were  the  surface  224 
feet  distant,  the  waves  produced  by  tw'o  syllables  could  go  and  return 
in  such  a time  as  to  be  heard  separately  from  the  direct  sound.  At 
560  feet,  flve  syllables  could  be  separately  heard. 

Very  sharp,  quick  souuds  produce  a less  permanent  effect  on  the  ear, 
and  can  therefore  cause  distinct  echoes,  when  the  reflecting  surface  is 
less  distant. 

Multiple  Echoes. — Wliea  the  source  of  sound  is  be- 
tween two  opposite  walls,  or  other  nearly  parallel  walls, 
the  waves  are  thrown  back  and  forth  between  the 
walls,  thus  repeating  the  original  sound  many  times. 
These  are  called  multiple  echoes. 

If  the  reflecting  surface  is  less  distant  than  112  feet, 
the  direct  and  reflected  sounds  are  blended,  and  but 
one  sound  is  heard.  If  the  direct  and  reflected  sounds 
reach  the  ear  at  nearly  the  same  time,  the  original 
sound  is  prolonged  and  strengthened,  otherwise  the 
two  sounds  produce  confusion. 

In  a properly  proportioned  room,  the  voice  of  a 
speaker  is  strengthened  by  the  waves  reflected  from 
the  walls  and  ceiling  reaching  the  ear  at  nearly  the 
same  time  as  the  direct  sound.  This  effect  is  spoken 
of  as  produced  by  the  resonance  of  the  room.  When, 
however,  the  hall  is  large,  the  reflected  sound  is  apt 
to  confuse  the  direct  sound  by  the  partial  echoes  so 
produced.  The  confusing  effects  caused  in  this  way 
in  large,  empty  halls,  are  greatly  diminished  when  the 
halls  are  crowded,  since  the  bodies  of  the  people  are 
bad  reflectors.  Curtains  and  drapery  have  a similar 
effect. 


13* 


150 


NATURAL  PHILOSOPHY. 


172.  Whispering  Galleries.  — If  two  carved  mir- 
rors, A and  B,  be  placed  facing  each  other,  as  shown 
in  Fig.  76,  and  anj  source  of  sound,  as,  for  example,  a 
watch,  be  suspended  at  a certain  distance  in  front  of 
the  mirror  A,  the  waves,  after  reflection  from  both 
mirrors,  Avill  collect  at  a point,  a,  in  front  of  the  mir-  ! 
ror,  B.  From  whatever  part  of  the  mirrors  the  waves  I 


Fig.  76,  — Eeflection  of  Sonnd-Waves. 

have  been  reflected,  they  will  all  reach  this  point,  a,  at 
the  sarne  time.  The  ticking  of  the  watch  can  there- 
fore be  distinctly  heard  by  a person  listening,  as  shown 
in  the  figure. 

Sometimes  the  ceilings  or  Avails  of  rooms  are  of 
such  a shape  that  a person  standing  in  certain  posi- 
tions ean  distinctly  hear  all  that  is  said  by  a person 
at  a distance  in  some  other  part  of  the  room,  and 
speaking  in  but  a faint  Avhisper.  The  shape  of  the 
ceiling  or  walls  is  such  that  the  sound-Avaves  reflected 
from  different  parts  are  all  brought  by  reflection  to 
the  place  Avhere  the  other  person  is  standing.  Eooms 
Avith  dome-shaped  ceilings  frequently  act  in  this  way. 
The  name  whispering  galleries  is  given  to  such  rooms. 


SOUND. 


151 


I In  the  dome  of  St.  Paul's,  in  London,  persons  standing  on  opposite 
j sides  of  the  gallery  can  converse  in  faint  whispers.  The  person  talking 
places  his  mouth  near  the  wall ; the  one  listening,  holds  his  ear  near 
i,  that  portion  of  the  wall  directly  opposite. 

' 173.  Refraction  of  Sound.  — The  straight  direc- 

tion in  which  sound-waves  move  is  changed  when  the 
waves  pass  from  one  medium  into  another  of  different 
density.  In  this  case,  the  direction  of  the  waves  is 
changed  at  the  surface  of  the  dense  medium,  but  the 
waves,  instead  of  being  thrown  off  or  reflected  by  the 
medium,  pass  through  it  in  a direction  somewhat  dif- 
ferent from  that  in  which  they  were  moving  before 
they  reached  the  other  medium.  The  change  caused 
in  this  manner  in  the  direction  of  sound  is  called  the 
refraction  of  sound.  Sound,  for  example,  is  refracted 
in  passing  from  air  to  water,  or  from  air  through  any 
solid,  or  the  reverse. 

174:.  The  Effect  of  Distance  on  Sound. — If  we  are 

walking  towards  a distant  bell  while  it  is  sounding,  we 
notice  that  the  sound  constantly  grows  louder  and 
louder  as  we  near  the  bell.  It  can  be  shown  that  the 
loudness,  or  the  intensity  of  any  sound,  decreases  as  the 
square  of  the  distance  from  the  source  of  the  sound. 
Thus,  if,  at  the  distance  of  ten  feet  from  the  bell,  we 
hear  the  sound  with  a certain  loudness  or  intensity  at 
twenty  feet,  or  twice  as  far  from  the  bell,  the  intensity 
would  be  found  to  be  but  one-fourth  as  great,  that  is, 
as  (if  or  = i. 

The  speaking -tules  lohich  connect  the  different  rooms 
in  a building,  enable  a person  talking  in  a moderate 
tone  to  be  distinctly  heard  by  a person  listening  at  an- 
other part  of  the  building.  Here  the  intensity  of  the 
sound  is  not  so  greatly  diminished,  because  the  sound- 
waves are  confined  to  the  air  in  the  tube,  and  so  do  not 


152 


NATURAL  PniLOSOFIIY. 


spread  outwards  in  all  directions.  The  faint  sounds, 
conveyed  by  the  string  or  the  wire  in  the  string-tele- 
phone are  distinctly  heard  at  the  other  end,  for  the 
same  reason. 

Experiment.  — Speak  faintly  into  an  empty  water-hose,  and  a per- 
son listening  at  the  other  end  can  hear  distinctly  all  that  is  said. 


Syllabus. 

The  word  sound  is  used  in  two  distinct  senses,  viz.:  1st.  As  the  im- 
pression produced  on  the  brain  by  sound-waves,  and  2d.  A.s  the  waves 
or  shakings  of  the  air  themselves. 

In  wave-motion,  the  particles  do  not  have  any  continued  onward 
motion  ; they  simply  move  backwards  and  forwards  through  compara- 
tively short  distances.  The  waves  themselves,  however,  move  onwards 
often  for  considerable  distances. 

A vibration,  wave,  or  undulation  consists  in  the  backward  and  for- 
ward motion  of  a particle  from  one  side  of  the  position  it  occupies 
when  at  rest  to  the  other  side. 

The  length  of  a wave  is  the  shortest  distance  between  any  two 
sets  of  particles  that  are  moving  at  the  same  time  in  the  same  direc- 
tion. 

The  amplitude  of  a wave  is  the  greatest  distance  any  particle  has 
been  moved  out  of  its  position  of  equilibrium  during  the  progress  of 
the  wave. 

The  wave-period,  or  the  time  of  vibration,  is  the  time  required  for 
any  particle  to  make  one  complete  movement  to  and  fro.  The  time 
of  vibration  is  the  same  whether  the  amplitude  is  great  or  small. 

Sound  is  caused  by  the  shakings  or  vibrations  of  some  body  ; there- 
fore all  bodies  producing  sound  are  in  vibration. 

Sound  is  conveyed  to  the  ear  by  waves,  called  sound-waves,  pro- 
duced in  the  surrounding  air  by  the  vibrating  body.  Sound-waves 
consist  of  alternate  condensed  and  rarefied  spaces  in  the  air  called 
waves  of  condensation  and  rarefaction. 

The  vibrating  body  strikes  the  air  near  it  and  sets  it  into  motion ; 
this  air  imparts  its  motion  to  the  air  next  to  it,  and  comes  to  rest,  and 
this  to  the  air  beyond  it,  until  the  last  particles  reach  the  ear  and  cause 
sound.  This  motion  is  similar  to  that  transmitted  through  a row  of 
glass  balls  touching  one  another,  when  a ball  is  thrown  against  one  of 
the  end  balls. 


QUESTIONS  FOR  REVIEW. 


153 


A medium  is  necessary  to  transmit  sound.  All  sounds  cease  in  a 
vacuum.  Any  elastic  substance  will  transmit  sound,  as,  for  example, 
water  and  most  solids. 

The  velocity  of  sound  in  air  at  the  temperature  of  freezing  water  is 
1090  feet  per  second.  Sound  travels  more  rapidly  in  hot  than  in  cold 
air.  Its  velocity  is  about  four  and  one-half  times  greater  in  water  than 
in  air.  Its  velocity  in  most  solids  is  greater  than  in  either  air  or  water. 

All  ordinary  sounds  travel  with  the  same  velocity. 

When  sound-waves  strike  any  large,  reflecting  surface,  they  are 
thrown  off,  or  reflected  from  it.  The  angle  of  reflection  is  equal  to 
the  angle  of  incidence. 

An  echo  occurs  whenever  sufficient  time  has  elapsed  between  the 
reflected  and  the  direct  sound  to  allow  us  to  hear  the  reflected  sound 
separately  from  the  direct  sound. 

In  whispering  galleries,  the  walls  or  ceilings  so  reflect  the  sound- 
waves as  to  bring  them  from  all  parts  of  the  room  to  nearly  the  same 
place  at  the  same  time. 

When  sound-waves  meet  a medium  whose  density  is  different  from 
that  in  which  they  have  been  moving,  they  are  turned  somewhat  out 
of  their  direction  on  entering  it.  This  is  called  the  refraction  of  sound. 

The  intensity  of  sound  is  inversely  proportional  to  the  square  of  the 
distance. 

Speaking-tubes,  wires,  and  strings  convey  sounds  distinctly  to  great 
distances,  because  the  motion  is  then  transmitted  in  only  one  direction. 

Questions  for  Review. 

In  what  two  different  senses  is  the  word  sound  used  ? 

Define  a vibration,  wave,  or  undulation.  What  is  meant  by  the 
length  of  a wave ? By  its  amplitude?  By  its  time  of  vibration ? Is 
the  time  of  vibration  affected  by  the  amplitude  ? 

In  wave-motion,  do  the  particles  actually  move  forwards  for  any 
considerable  distance?  What  is  the  true  nature  of  their  motion  ? 

What  is  the  cause  of  sound  ? Describe  any  experiment  which  shows 
that  a sounding  body  is  in  vibration. 

How  is  the  motion  of  the  vibrating  body  conveyed  to  the  ear,  in 
order  to  produce  the  sensation  of  sound  ? 

Describe  the  nature  of  sound-waves.  What  name  is  given  to  these 
waves?  In  what  respects  is  the  motion  of  sound-waves  through  air, 
or  any  other  body,  like  the  motion  sent  through  a row  of  glass  balls 
touching  one  other,  when  one  of  the  end  balls  is  struck  ? 


154 


NATURAL  PHILOSOPHY. 


Why  is  a medium  necessary  to  transmit  sound  ? Will  any  elastic 
medium  transmit  sound  ? Describe  any  experiment  which  proves  that 
strings  or  wires  transmit  sound. 

What  is  the  velocity  of  sound  in  air  ? How  is  the  velocity  of  sound 
in  air  affected  by  the  temperature  ? Have  all  ordinary  sounds  the 
same  velocity  in  air  ? How  can  you  prove  this  ? How  does  the  veloc- 
ity of  sound  in  water,  or  in  elastic  solids,  compare  with  its  velocity  in 
air? 

How  does  the  angle  of  reflection  compare  with  the  angle  of  inci- 
dence ? 

What  IS  the  cause  of  echoes  ? How  far  must  the  reflecting  surface 
be  from  the  source  of  sound  in  order  to  give  an  echo  of  a single  syl- 
lable of  articulate  speech?  What  is  the  cause  of  multiple  echoes? 
What  are  whispering  galleries  ? 

What  is  meant  by  the  refraction  of  sound?  What  is  the  difference 
between  the  reflection  and  the  refraction  of  sound  ? 

What  effect  is  produced  on  the  intensity  of  sound  by  the  distance 
from  the  source  of  sound  ? 

Why  can  sounds  be  carried  so  much  further  through  speaking-tubes, 
or  through  wires  or  strings,  than  through  the  open  air  ? 


CHAPTER  11. 

THE  CHARACTERISTICS  OF  MUSICAL 
SOUND.— MUSICAL  INSTRUMENTS. 

175.  Musical  Sound  and  Noise. — When  the  vibra- 
tions of  the  sounding  body  are  regular  and  periodical, 
that  is,  when  the  time  of  each  vibration  is  the  same, 
a musical  sound  is  produced ; but  when  the  time  of 
vibration  is  irregular,  a noise  is  heard.  Noises  may  be 
momentary^  that  is,  may  produce  an  effect  on  the  ear 
of  too  short  a duration  to  be  measured,  as,  for  example, 
the  report  of  a gun ; or  they  may  be  continuous,  such 
as  those  caused  by  the  rolling  of  thunder,  or  the  saw- 
ing of  a board ; continuous  noises  are  produced  by  a 
mingling  of  many  discordant  musical  sounds. 

The  unpleasant  effect  produced  by  noises  on  the  ear  has  been  com- 
pared to  a similar  effect  produced  by  a flickering  light  on  the  eye. 
In  both  cases,  the  unpleasant  feeling  is  most  probably  caused  by  the 
sudden  and  abrupt  changes  which  these  organs  transmit  to  the  brain. 

176.  Musical  Sounds  produced  by  Regular  Im- 
pulses.— That  regular  impulses  will  produce  musical 
sounds  when  they  follow  each  other  with  sufficient 
rapidity,  is  a matter  of  common  experience.  As  each 
tooth  in  a circular-saw  strikes  the  board  it  is  sawing,  a 
momentary  sound  is  produced ; but  if  the  saw  is  mov- 
ing with  sufficient  rapidity,  a more  or  less  musical 
note  is  heard. 


155 


156 


NATURAL  PHILOSOPHY. 


Experiment — Draw  the  blade  of  a penknife  over  the  milled  edge 
of  a large  coin ; if  the  motion  be  sufficiently  rapid,  the  separate  taps 
will  produce  a musical  tone. 

177.  The  Characteristics  of  Musical  Sounds. — A 

very  little  observation  will  convince  us  that  all  sounds 
are  not  the  same.  They  differ  from  one  another  in  a ' 
variety  of  ways.  These  differences  may,  however,  all  ^ 
be  traced  to  three  peculiarities  or  characteristics,  viz.,  i 
the  intensity.,  the  pitch,  and  the  quality. 

178.  Intensity.  — By  the  intensity  of  a soxind  we  ■ 
mean  the  peculiarity  that  enables  us  to  distinguish  be- 
tween tones  that  are  loud  or  feeble.  The  intensity  of  a i 
sound  is  dependent  on  the  amplitude  of  the  waves  pro- 
ducing it,  which,  in  sound-waves,  depends  upon  the 
extent  to  which  the  air  is  alternately  condensed  and  , 
rarefied  by  the  vibrating  body,  or  upon  the  degree  of 
rarefaction  and  condensation  that  exist  in  the  sound- 
waves which  reach  the  ear. 

When  a bell  is  struck  vigorously,  it  gives  a loud  sound,  because  the 
distance  through  which  its  sides  swing  is  comparatively  great,  and 
the  air  around  the  bell  is  considerably  condensed  or  rarefied. 

Experiment Stretch  a wire  firmly  between  two  stout  hooks  se-  , 

curely  fastened  to  the  top  of  a table.  Pluck  the  wire  gently  with  the 
fingers,  and  a musical  note  will  be  heard.  Now  pluck  the  wire  at  the 
same  point,  but  with  more  force,  and  a note  will  be  heard  louder  than 
before,  but  neither  higher  nor  lower.  The  two  notes  differ  in  their 
intensity  or  loudness. 

Caution. — The  wire  must  be  quite  tight.  If  a stout  screw  can  be 
obtained,  the  wire  may  be  fastened  to  the  hook  at  one  end,  and  the 
other  end  be  secured  to  the  screw,  and  tightened  by  moving  the  screw 
in  the  proper  direction. 

179.  The  Speaking-Trumpet  is  used  to  allow  the 
voice  to  be  heard  at  great  distances.  It  is  conical  in 
shape,  and  has  a trumpet-shaped  end.  The  small  end 
is  held  to  the  mouth  of  the  person  talking. 


CHARACTERISTICS  OF  MUSICAL  SOUND.  157 


Experiment.  — Roll  a piece  of  stout  pasteboard  into  a cone;  place 
the  mouth  at  the  small  end,  and  talk  or  sing  into  it.  The  voice  will 
be  greatly  strengthened.  Point  the  cone  directly  at  a person  standing 
in  the  far  end  of  a room,  and  whisper ; he  will  he  able  to  hear  dis- 
tinctly all  that  is  said,  while  those  on  either  side  of  the  instrument, 
but  nearer  it,  are  unable  to  hear. 


180.  The  Ear-Trumpet  is  used  to  aid  deaf  per- 
sons in  hearing.  It  acts  bj  concentrating  the  voice 
on  the  listener’s  ear.  Its  shape  is  similar  to  that 
of  the  speaking-trumpet,  only  the  small  end  is  placed 
in  the  ear,  and  the  person  talks  into  the  large  end. 

Experiment. — Place  the  small  end  of  the  paper  cone  used  in  the 
preceding  experiment,  in  a person’s  ear  and  whisper  into  the  large  end, 
and  if  no  defect  of  hearing  exists,  he  will  hear  distinctly.  Go  to  the 
far  end  of  the  room  and  again  whisper,  though  somewhat  louder,  and  he 
will  still  hear  what  is  said. 

181.  Pitch.  — By  the  ijitch  of  a musical  sound  we 
mean  that  peculiarity  that  enables  us  to  distinguish 
between  sounds  that  are  high  or  ?ow,  sharp  or  grave. 

The  pitch  depends  on  the  number  of  vibrations 
per  second  imparted  by  the  sounding  body  to  the  air. 
The  greater  the  number  of  vibrations,  the  higher  the 
pitch  of  the  sound,  that  is,  the  shriller  the  note.  Thus, 
when  a circular-saw  is  in  motion,  the  sound  it  causes  is 
shriller  the  more  rapid  its  revolution. 

A wheel  furnished  with  teeth  on  its 
circumference,  and  supported  on  a suit- 
able frame,  as  shown  in  Fig.  77,  may 
be  set  in  rapid  rotation  by  means  of 
a cord  wrapped  around  its  axis.  If 
a card  be  now  held  against  the  teeth, 
a musical  sound  will  be  produced,  the  Pig.  77.— Savait’s 

pitch  of  which  will  be  shriller  the  Wheel, 

more  rapid  the  rotation.  As  the  wheel  gradually 
14 


158 


NA  TURAL  PHIL  OSO PH Y. 


loses  its  motion,  and  moves  slower  and  slower,  tlie 
pitch,  of  the  note  changes  in  a marked  manner. 

The  shorter  the  length  of  a string  or  wire,  the  more 
rapid  its  vibration.  The  shrill  treble  notes  of  a piano 
are  produced  by  the  short,  thin  strings ; the  grave  base 
notes,  by  the  long,  thick  strings. 

After  the  strings  of  a piano  have  been  struck,  the  sound  will  grow 
fainter  and  fainter,  because  the  amplitude  of  the  vibration  of  the 
string  grows  less  and  less.  The  pitch  of  the  note,  however,  will  not 
change,  since  the  time  of  vibration  is  the  same,  whatever  may  be  the 
amplitude  of  the  vibration. 

Experiment. — Cut  out  a piece  of  hard  wood,  with  a flat  base  and  a 
sharp  edge,  rather  too  high  to  go  under  the  wire  stretched  across  the 
top  of  the  table,  as  described  in  a previous  experiment.  Lift  the 
wire  and  place  the  piece  of  wood  under  it,  so  that  the  wire  will  press 
firmly  against  the  sharp  edge.  Now,  by  sliding  the  piece  of  wood  from 
one  end  of  the  wire  to  the  other,  and  vibrating  the  wire  between  the 
wood  and  either  end  of  the  wire,  it  will  be  found  that  the  shorter  the 
portion  of  wire  vibrated,  the  shriller  will  be  the  note  it  gives,  that  is, 
the  higher  its  pitch. 

Experiment. — The  shrill  sound  made  on  a slate  or  black-board  by 
the  pencil  or  chalk  is  caused  by  a series  of  taps  rapidly  following  one 
another.  The  taps  can  be  recorded  on  the  black  board  as  follows. 
Select  a long  crayon,  and  holding  it  loosely  in  the  fingers  at  about  the 
middle  of  the  crayon,  move  it  in  an  arc  over  the  black-board,  so  as 
to  cause  it  to  emit  a shrill  sound.  Examine  the  curved  line,  and  it 
will  be  seen  to  consist  of  a number  of  separate  marks  made  each  time 
the  chalk  tapped  against  the  board.  By  altering  the  pressure  of  the 
chalk  on  the  board,  sounds  of  different  pitch  may  be  obtained ; and  on 
examining  the  line,  it  will  be  found  that  the  shriller  sounds  were  those 
produced  by  the  greatest  number  of  taps  in  a given  time. 

Caution. — To  succeed  with  this  experiment,  some  little  practice  may 
be  necessary.  It  is,  however,  so  beautiful  and  instructive,  that  it 
should,  if  possible,  be  shown. 

182.  The  Limits  of  the  Ear. — Though  the  limits 
of  hearing  vary  somewhat  in  different  persons,  none 
can  hear  sounds  produced  hy  fewer  than  16  vibrations 
per  second,  or  more  than  about  -±8,000  per  second.  If 


CHARACTERISTICS  OF  MUSICAL  SOUND.  159 


the  vibrations  are  fewer  than  16  per  second,  we  hear 
each  as  a separate  tap,  or  puff ; if  they  are  more  than 
about  48,000,  the  sound  becomes  too  shrill  to  be 
audible. 

183.  The  Siren, — -The  exact  number  of  vibrations 
necessary  to  produce  a given  note  can  be  determined 
by  means  of  an  instrument  called  the  siren. 

A sectional  view  of  this  instrument  is  shown  in  Fig.  78.  The  cylin- 
der, A,  is  mounted  on  a wind-chest.  At  the  upper  end  of  the  cylinder 
is  fixed  a smooth,  metallic  plate,  E.  Immediately  above  the  fixed 
plate,  E,  is  a movable  plate,  C,  attached  to  an 
axis,  D,  which  permits  it  to  move  freely  over  the 
plate,  E.  Both  E and  C are  pierced  at  regular 
intervals  with  small  holes,  extending  through 
the  plates  in  an  oblique  direction,  as  shown  in 
the  figure.  The  holes  in  E are  equal  in  number 
to  those  in  C,  but  are  inclined  in  the  opposite 
direction. 

When  a current  of  air  is  forced  through  the 
cylinder,  A.  the  plate,  C,  and  the  axis  to  which 
it  is  connected,  are  set  in  rapid  rotation,  and  a 
musical  note  is  produced,  whose  pitch  increases 
as  the  speed  of  rotation  becomes  greater.  This 
note  is  caused  by  the  columns  of  compressed  air 
that  are  allowed,  at  regular  intervals,  to  escape 
through  the  openings  of  the  plate,  E,  when  they 
are  not  closed  by  the  plate,  C.  The  number  of  columns  of  compressed 
air  that  escape  in  this  manner  will,  of  course,  depend  on  the  number 
of  openings  in  the  plate  E.  and  the  speed  of  revolution  of  C. 

To  determine  the  number  of  revolutions  of  C,  an  endless  screw,  H, 
attached  to  the  axis,  D,  moves  counters  (like  those  on  gas-meters),  over 
graduated  dials,  by  means  of  the  toothed  wheels,  0 and  1. 

In  order  to  use  the  siren  to  ascertain  the  pitch  of  any  note,  wind  is 
urged  through  the  cylinder.  A,  until  the  pitch  of  the  note  given  by  the 
siren  is  the  same  as  that  of  the  note  the  number  of  whose  vibrations 
is  to  be  determined.  The  speed  of  the  siren  is  now  kept  constant, 
and  the  number  of  revolutions  of  the  plate  during  any  second  ascer- 
tained. This,  multiplied  by  the  number  of  openings  in  the  plate,  E, 
will  give  the  number  of  vibrations  per  second  required  to  produce  the 
given  note. 


t 

Fig.  78. — The  Siren. 


160 


NATURAL  PHILOSOPHY. 


184:.  The  Quality. — When  we  sound  the  same  note 
with  equal  loudness  on  two  different  musical  instru- 
ments, as,  for  example,  on  a piano  and  on  a flute,  al- 
though the  notes  are  of  the  same  pitch  and  intensity, 
yet  there  is  something  which  enables  the  ear  to  dis-  i 
tinguish  one  note  from  the  other.  So,  when  two  per-  I 
sons  are  speaking  in  the  same  tone,  we  can  recognize  a ' 
difference  in  the  sounds  produced  ; these  peculiarities  ! 
are  known  as  the  quality  of  the  sounds.  \ 

185.  Cause  of  Differences  of  Quality. — When  sin- 
gle notes  are  produced  by  any  musical  instruments,  we 
are  generally  able  to  hear  but  a single  sound,  that  is,  a 
sound  of  a given  pitch.  In  nearly  all  cases,  however, 
accompanying  the  sound  of  the  note  struck,  are  a 
number  of  other  sounds  of  different  pitch,  but  of  so 
feeble  intensity  that,  unless  we  listen  attentively,  we 
are  unable  to  distina-uish  them.  These  additional 

O 

tones  are  called  overtones.  Although  we  do  not  hear 
them  as  separate  tones,  yet,  mingling  with  the  tone  of  ‘ 
the  note  struck,  they  impart  to  it  a peculiarity  of  tone 
which  we  recognize  as  its  quality. 

Differences  of  quality  are  caused  hy  differences  in  the 
pitch  of  the  overtones,  or  in  their  intensity. 

186.  Sympathetic  Vibrations. — Partially  raise  the 
top  of  a piano,  and  place  the  foot  on  the  loud  pedal; 
lean  over  the  instrument,  and  sing  any  note  into  it  in 
a loud  voice.  On  ceasing  to  sing,  we  wdll  bear  the 
same  note  given  back  by  the  piano.  Now  sing  a dif- 
ferent note,  and  it  will  be  found  that  the  piano  will 
give  back  this  particular  note,  and  so  with  others. 
The  sound-waves  produced  by  the  voice  have  struck 
against  all  the  strings  of  the  piano,  but  have  only  set  ' 
in  vibration  those  particular  strings  that  are  capable 


CHARACTERISTICS  OF  MUSICAL  SOUND.  161 


of  giving  sounds  of  the  same  pitch  as  their  own. 
Vibrations  so  produced  are  called  syrapathetic  vibra- 
tions. 

The  cause  of  sympathetic  vibration  is  as  follows  : The  sound-waves 
strike  all  the  strings,  and  give  each  a very  feeble  push.  If,  now, 
the  time  of  vibration  of  any  string  be  exactly  the  same  as  the  time  of 
vibration  of  the  sound-wave,  the  next  forward  impulse  which  the 
waves  give  to  the  string  will  be  received  by  it  at  exactly  the  time  when 
it  is  beginning  to  move  forward,  and  hence  the  motion  it  acquired  by 
the  first  impulse  is  increased  by  the  second  impulse,  and  as  the  same 
is  true  of  all  the  other  impulses,  the  string  at  last  acquires  a consider- 
able motion,  and  emits  an  audible  sound.  If,  however,  the  string  has 
a rate  of  motion  different  from  that  of  the  sound-waves,  it  will  some- 
times receive  a forward  impulse  from  the  waves,  when  it  is  moving  in 
the  opposite  direction,  and  its  motion  being  thereby  diminished,  it  can 
never  acquire  any  very  considerable  motion. 

Experiment.  — Suspend  a heavy  weight,  of  say  10  or  15  lbs.,  by  a 
string,  and  let  it  swing  as  a pendulum.  Note  the  time  of  its  oscilla- 
tion. Now,  while  it  is  swinging  very  gently,  blow  a puff  of  air  against 
it  from  the  mouth  just  as  it  is  moving  away.  Wait  until  it  is  again 
moving  away,  and  give  it  another  puff  of  air.  Do  this  thirty  or  forty 
times,  and  the  pendulum  will  acquire  a considerable  increase  of 
motion. 

While  the  pendulum  is  swinging  freely,  give  it  the  puffs  of  air  when 
it  is  just  beginning  to  move  towards  you,  and  the  motion  will  be  stopped 
sooner  than  otherwise. 

Caution. — As  regular  and  somewhat  forced  breathing  is  apt  to 
produce  headache,  a pair  of  hand-hellows  may  advantageously  replace 
the  breath. 

187.  Examples  of  Sympathetic  Vibrations. — 

Stand  near  a table  on  which  a number  of  goblets  and 
glasses  of  different  sizes  are  placed  upright.  Sing 
powerfully  a number  of  different  notes.  If  the  proper 
note  be  struck,  a goblet  or  glass  will  give  it  back 
again. 

Two  clocks,  whose  pendulums  are  exactly  of  the 
same  length,  are  hung  on  the  same  wall.  One  clock  is 
started  and  the  other  stopped.  The  movement  of  the 
14*  L 


162 


NATURAL  PUILOSOFIIY. 


pendulum  of  the  one  may,  it  is  said,  at  last  move  the 
pendulum  of  the  other  sufficiently  to  start  the  clock. 

The  gas-lights  in  a ball-room  have  been  known  to  ] 
flicker  when  in  the  music  of  the  orchestra  certain  j 
notes  were  sounded.  ! 

188.  Resonance. — We  have  already  seen  how  the  * 
voice  of  a speaker  is  strengthened  by  the  reflection  i 
of  sound-waves  from  the  ceiling  and  walls,  or,  as  it  i 
is  called,  Ijy  the  resonance  of  the  room.  The  effect  of  \ 
resonance,  however,  is  more  marked  when  the  sound-  j 
waves  are  enabled  to  set  in  vibration  some  elastic 
body  whose  time  of  vibration  is  exactly  the  same  as  j 
their  own. 

The  strings  of  a violin  or  guitar  are  too  thin  to  set  | 
much  air  in  motion.  The  notes  produced  by  these  J 
instruments  are  really  due  to  vibrations  of  the  wood  i 
forming  the  body  of  the  instruments;  and  it  is  on  the  i 
elasticity  of  this  wood,  and  its  ability  to  accept  from  j 
the  strings  different  rates  of  motion,  that  the  value  of  j 
the  instrument  depends.  It  would  be  found  on  trial  i 
that  a wire  stretched  across  the  corner  of  a room  to  the  i 

I 

walls,  would  give  a much  feebler  sound  than  if  it  were 
attached  to  hooks  in  the  top  of  a table,  because  the  table 
can  take  up  the  motion  of  the  wire  much  better  than  j 
the  walls  of  the  room.  i ; 

A mass  of  air  whose  dimensions  are  such  as  to  cn-  j 
able  it  to  vibrate  in  exactly  the  same  time  as  a certain  j 
sound,  will  be  set  in  motion  by  the  sound,  and  greatly  || 
increase  its  intensity  by  its  resonance.  Resonance  of  jj 
this  kind  is,  therefore,  dependent  for  its  action  on  li 
sympathetic  vibrations.  i ! 

In  order  to  increase  the  intensity  of  the  overtones  | i 
of  a note  sufficiently  to  enable  them  to  be  distinctly 


CHARACTERISTIC S OF  MUSICAL  SOUND.  163 


heard,  instruments  called  resonators  are  employed. 
They  consist  of  hollo^v  spheres  of 
brass,  of  the  shape  shown  in  Fig.  79, 

Avith  openings  at  a and  h ; one  of  these, 
n,  is  placed  in  the  car,  and  at  the  other 
the  sound-waves  enter.  If  there  be 
present  in  the  tone  an  overtone  whose  l'ig'79'“E,esoiiator. 
rate  of  vibration  is  exactly  the  same  as  the  rate  in 
Avhich  air  contained  in  the  sphere  can  best  vibrate,  the 
resonance  of  the  sjDhere  will  cause  the  sound  to  be  dis- 
tinctly heard.  The  resonant  case  on  which  a tuning- 
fork  is  mounted,  should  contain  a column  of  air  whose 
rate  of  vibration  is  exactly  that  of  the  fork. 

Across  the  end  of  the  tube  leading  from  the  outer 
air  into  the  ear  is 
a tightly  stretched 
drum-head,  called 
the  tympanum. 

The  column  of  air 
contained  within 
this  tube  is. capa- 
ble of  resound- 
ing to  and  greatly 
strengthening  cer- 
tain sounds  by  res- 
onance, as  is  par- 
tially illustrated 
by  the  following 


Experiment Tie  two 

strings  to  a poker,  or  a 
bar  of  iron  or  steel, 
some  little  distance  from 
the  ends,  as  shown  in  Fig.  80.  — An  Experiment  in  Eesonance, 

Fig.  80.  Hold  the  ends  of  the  string  over  the  end  of  one  of  the 
fingers  of  each  hand,  letting  the  poker  hang  in  a horizontal  position. 


164 


NATURAL  P [I ILOSOrUY. 


Now  insert  in  the  ears  the  fingers  holding  the  strings,  being  careful  toj  J 
avoid  allowing  the  strings  to  rest  against  the  body.  Strike  one  end 
of  the  poker  against  the  wall,  or  have  some  person  strike  it  near  the' 
middle,  and  a sound  will  he  heard  like  that  of  a larrje  hell,  when  you 
are  very  near  it.  Here  the  vibrations  of  the  poker  are  transmitted 
through  the  strings  to  the  columns  of  air  within  the  ears,  which  by 
resonance  strengthen  the  sound.  The  effect  is  to  a great  extent  due  to 
the  vibrations  being  carried  through  the  bones  of  the  head  directly  to 
the  ear. 

Caution. — Allow  the  whole  weight  of  the  poker  to  be  supported  by 
the  string.  Avoid  having  the  string  partly  supported  by  the  ear.  ^ 

The  air  which  fills  any  hollow  body  will  respond  to  J 
some  particular  note.  When  a shell  is  held  to  the  ear 
a sound  is  heard,  which  a pretty  superstition  of  child- 1 

hood  regards  as  thef 
imprisoned  sounds  of  | 
ocean  waves  beating  * 
against  a shore.  The 
sounds  are  really 
caused  by  the  air  with- 1 
in  the  shell  strength-  ' 
euing  by  its  resonance  *■ 
the  feeble  sounds  that, 
are  always  present  in  i 
the  air.  Similar  sounds  i 
may  be  heard  by  hold- 
ing an  empty  pickle- 1 
iar  or  tomato-can  near  | 
the  ear. 

Experiment. — The  follow-  ^ 
ing  piece  of  apparatus,  well  * 
known  to  most  boys,  mav  be  1 
Fig,  81,-Aa  Experiment  in  Eesonance.  follows:'  Punch  a| 

hole  in  the  bottom  of  an  empty  tomato-can.  Kun  a stout  stringl 
through  the  hole,  and  knot  the  string  at  the  end  to  prevent  its  being J 
pulled  out.  Holding  the  can  in  the  hand,  as  shown  in  Fig.  SI.  run  the 
rosined  fingers  down  the  string,  and  a noise  far  from  musical  will  be 


CHARACTERISTICS  OF  MUSICAL  SOUND.  165 


heard.  The  vibrations  of  the  string  are  strengthened  by  the  resonance 
of  the  air  within  the  can. 

Try  the  same  experiment  with  a shorter  can,  and  it  will  be  found  to 
give  a much  shriller  note. 

189.  The  Interference  of  Sound-Waves. — When 
two  notes  of  the  same  intensity  and  pitch  are  sounded 
together,  they  sometimes  strengthen  and  sometimes  com- 
pletely obliterate  each  other. 

That  two  sounds  could  he  so  put  together  as  to  cause 
silence.^  seems,  at  first  thought,  impossible ; but  if  Ave 
remember  that  sound  is  an  effect  produced  by  a Avave- 
motion,  aa'C  can  easily  see  Iioav  one  sound  could  oblit- 
erate another,  for  if  one  sound-AA^ave  is  endeavoring 
to  condense  the  air  at  the  same  moment  the  other  is 
endeavoring  to  rarefy  it,  the  air  AV'ould  neither  be  con- 
densed nor  rarefied,  and  silence  aa'quH  result.  If,  hoAV- 
ever,  each  sound  rarefied  or  condensed  the  air  at  the 
same  time,  the  degree  of  rarefaction  or  condensation 
would  be  increased,  and  the  sound  Avould  be  louder. 
These  effects  are  knoAAm  as  the  interference  of  sound- 
icaves. 

190.  Musical  Instruments. — There  are  three  classes 
into  Avhich  musical  instruments  may  be  divided,  viz., 
stringed  instruments.,  icind  instruments,  and  instruments 
in  which  the  sounds  are  produced  by  the  vibration  of 
plates  or  membranes. 

1.  Stringed  Instruments.  — Examples  of  stringed  in- 
struments are  seen  in  the  piano,  harp,  violin,  violincello, 
guitar,  and  banjo.  In  the  piano  and  harp,  there  is  a 
separate  string  for  each  note.  In  each  of  the  other 
instruments,  the  same  string  is  made  to  give  different 
notes,  by  touching  it  at  different  points  AA’ith  the  fin- 
ger, and  thus  practically  shortening  its  length.  The 
tighter  any  string  is  stretched,  the  shorter  its  length, 


166 


NATURAL  PHILOSOPHY. 


and  the  smaller  its  diameter  the  shriller  the  note  which 
it  will  give. 

2.  Whid  Instruments.  — The  sounds  produced  hv 
wind  instruments  are  caused  by  the  vibration  of  a 
column  of  air  contained  within  the  instrument.  The 
pitch  of  the  note  is  dependent  upon  the  dimensions  of 
the  air  column,  especially  its  length,  and  also  upon 
whether  the  tube  containing  the  column  of  air  is  open 
at  both  ends,  or  at  but  one  end.  The  column  of  air 
within  the  instrument  may  be  set  into  vibration  in  a 
number  of  ways,  of  which  we  shall  consider  two  of  the 
most  important,  viz. ; 1st.  By  means  of  a mouth-piece, 
and  2d.  By  means  of  a vibrating  plate  called  a reed. 

In  the  organ-pipe,  the  vibration  of  the  air  column 
is  produced  by  the  action  of  a mouth-piece.  The 
organ-pipe  is  placed  on  a box  called  the  wind- 
chest,  supplied  with  air  from  a bellows.  The  air 
entering  through  the  opening,  a,  Fig.  82,  passes 
through  a narrow  slit,  c,  and  escapes  at  the  open- 
ing, 0.  The  air  does  not  flow  out  of  this  opening 
in  a continuous  stream  ; for,  on  striking  against 
the  bevelled  lip,  r,  it  is  broken  into  a flutter  or 
series  of  puffs,  the  rate  of  which  is  controlled  by 
the  length  and  size  of  the  column  of  air  in  the  pipe. 

A reed  is  a thin,  vibrating  plate  of  any  elas- 
tic material,  which  is  moved  backwards  and  for- 
wards by  the  air.  The  notes  of  reed-organs, 
accordions,  and  Jews-harps  are  caused  b}'  the 
vibrations  of  reeds. 


Fig.  82, 
— An 
Organ- 
Pipe. 


I 

! 

1 

I' 


h 


Experiment. — Cut  a stout  wheaten  straw  into  a length  of  about 
four  inches  from  the  knot.  With  a sharp  penknife,  cut  a slit,  a,  down 
to  the  knot,  h,  as 

t shown  in  Fis;.  S3.  Ivow 
Fig.  83.  — A Straw  Seed.  i b 

“ place  the  mouth  completely 

over  the  cut  part  and  blow,  and  a musical  note  will  be  heard.  The 


CHARACTERISTICS  OF  MUSICAL  SOUND.  167 


pitch  of  this  note  will  increase,  if  the  length  of  the  straw  tube  is  short- 
ened by  cutting  a piece  off  the  open  end. 

Caution. — If  the  pipe  will  not  give  a note  at  once,  do  not  throw  it 
away.  A certain  pressure  with  which  the  air  is  driven  through  it  is 
necessary  in  order  to  make  the  pipe  sound. 

Experiment. — Prepare  a straw  reed  as  before,  only  use  a longer 
straw,  and  cut  holes  in  the  side  about  one  inch  apart.  Sound  the 
reed  when  the  holes  are  all  open,  and  remember  the  pitch  of  the 
note.  Now,  with  one  finger  close  the  hole  nearest  the  knot  end,  and 
the  pitch  of  the  note  jirodueed  will  he  lower:  close  the  next  hole, 
still  keeping  the  finger  on  the  opening  previously  closed,  the  note  will 
be  still  lower.  These  effects  are  the  same  as  would  be  produced  by 
lengthening  the  pipe.  Hcnec,  we  may  conclude  that  in  any  wind  instru- 
ment, the  length  of  the  vibrating  column  is  affected  by  openings  in  the 
side  of  the  instrument. 

Caution. — Blow  with  as  nearly  the  same  force  in  each  case  as  possi- 
ble, as  different  notes  are  produced  in  the  same  pipe  by  blowing  with 
different  degrees  of  strength. 

In  the  flute,  flageolet,  and  fife,  the  different  notes  are  produced  by 
virtually  altering  the  length  of  the  air  column  by  opening  or  closing 
holes  in  the  side  of  the  instrument. 

3.  Musical  Instruments  whose  notes  are  due  to  the 
vibrations  of  plates  or  membranes. 

In  the  musical-box,  the  notes  are  produced  by  the 
vibrations  of  a series  of  steel  plates  of  different  lengths, 
which  are  set  into  motion  by  pins  projecting  above  the 
surface  of  a revolving  cylinder. 

In  the  xylophone,  the  notes  are  produced  by  the 
vibrations  of  plates  of  wood  of  different  lengths.  The 
notes  of  the  cymbal  are  produced  by  the  vibrations  of 
brass  plates  ; those  of  the  drum,  by  the  vibrations  of  a 
membrane. 

Syllabus. 

The  vibrations  which  produce  a musical  sound,  follow  one  another 
at  regular  intervals  of  time ; those  which  produce  noise,  follow  one 
another  at  irregular  intervals. 


168 


NATURAL  PHILOSOPHY. 


I 


The  unpleasant  effect  produced  on  the  car  by  noisy  sounds  is  similar 
to  that  produced  on  the  eye  by  a flickering  light. 

There  are  three  characteristics  of  musical  sound,  viz.,  intensity  or 
loudness,  pitch,  and  quality. 

The  intensity  of  the  sound  is  that  peculiarity  in  virtue  of  which 
sounds  are  loud  or  feeble.  The  intensity  of  a sound  is  dependent  on 
the  amplitude  of  the  vibrations  causing  it. 

By  the  amplitude  of  a sound-wave,  we  mean  the  degree  of  condensa- 
tion or  rarefaction  in  the  wave. 

The  speaking-trumpet  is  used  to  enable  tbe  sound  of  the  voice  to  be 
heard  at  a great  distance.  The  ear- trumpet  is  used  to  aid  deaf  persons 
in  hearing. 

By  the  pitch  of  a tone,  we  mean  the  peculiarity  that  enables  us  to 
distinguish  between  sounds  that  are  high  or  low,  sharp  or  grave.  The 
pitch  of  a sound  depends  on  the  number  of  vibrations  per  second,  that 
is,  on  the  rapidity  with  which  the  sounding  body  is  vibrating. 

We  cannot  hear  sounds  that  are  produced  by  fewer  than  16  or  more 
than  48,000  vibrations  per  second. 

By  the  qnality  of  a musical  sound  is  meant  that  peculiarity  which 
enables  us  to  distinguish  between  two  notes  of  the  same  pitch  and  in- 
tensity when  sounded  on  different  instruments.  The  differences  in  the 
quality  of  musical  sounds  are  caused  by  the  differences  in  the  pitch  and 
intensity  of  the  overtones  which  accompany  them. 

Sympathetic  vibrations  are  vibrations  of  the  same  period  that  are 
produced  in  surrounding  bodies  by  sound-waves. 

Sounds  are  often  strengthened  by  the  resonance  of  elastic  substances 
in  which  the  sound-waves  produce  sympathetic  vibrations.  The’  notes 
given  by  the  strings  of  a violin  or  guitar  are  greatly  strengthened  by 
the  resonance  of  the  wood  forming  the  body  of  these  instruments. 

The  column  of  air  within  the  tube  of  the  human  ear  strengthens 
certain  sounds  by  its  resonance. 

The  confused  murranrings  heard  when  an  empty  shell  is  held  to  the 
ear  are  caused  by  the  faint  noises  present  in  the  air  being  strength- 
ened by  the  resonance  of  the  air  within  the  shell. 

By  the  interference  of  sound-waves,  we  mean  the  strengthening  or 
weakening  of  the  amplitude  of  one  wave  by  another. 

Musical  instruments  may  be  divided  into  stringed  instruments,  wind 
instruments,  and  those  in  which  the  notes  are  produced  by  tbe  vibration 
of  plates  or  membranes.  The  piano,  harp,  guitar,  violin,  violincello, 
and  banjo  are  examples  of  stringed  instruments. 

The  notes  of  wind  instruments  are  caused  by  the  vibrations  of  a 
column  of  air  contained  within  the  body  of  the  instrument.  This 
column  may  be  set  into  vibration,  1st.  By  a mouth-piece ; 2d.  By  a reed. 


QUESTIONS  FOR  REVIEW. 


169 


An  organ-pipe  is  a good  example  of  a musical  instrument  where  the 
air  is  set  in  vibration  by  a mouth-piece.  In  the  accordion,  melodeon, 
and  Jews-harp,  the  sound  is  caused  by  a reed.  The  musical-box,  xylo- 
phone, cymbal,  and  drum  are  examples  of  the  third  class. 


Questions  for  Review. 

What  is  the  difference  between  musical  sounds  and  noises  ? What 
are  the  three  characteristics  of  musical  sounds  ? 

Define  intensity  or  loudness.  By  what  are  differences  in  the  in- 
tensity or  loudness  of  sounds  caused  ? Describe  any  experiment  by 
means  of  which  variations  in  the  intensity  of  sounds  can  be  shown. 

Describe  the  speaking- and  ear-trumpets.  For  what  is  each  used? 
How  may  some  simple  experiments  with  these  instruments  be  shown  ? 

Define  pitch.  By  what  are  differences  in  the  pitch  of  musical  sounds 
caused  ? 

What  is  the  gravest  sound  the  human  ear  can  hear  ? What  is  the 
shrillest  ? 

How  may  the  sound  produced  by  rubbing  a piece  of  chalk  on  a 
black-board  be  made  to  record  its  vibrations  ? Describe  the  siren. 

Define  quality.  By  what  are  differences  in  the  quality  of  musical 
sounds  caused?  What  is  meant  by  overtones  ? 

Explain  in  full  what  is  meant  by  sympathetic  vibrations.  Give 
some  examples  of  sympathetic  vibrations. 

How  may  the  strength  of  any  sound  be  increased  by  resonance  ? 
What  has  the  body  of  a violin  or  guitar  to  do  with  the  strengthening 
of  the  sounds  of  the  strings  ? 

Describe  any  experiment  which  shows  the  power  of  the  column  of 
air  within  the  ear  to  greatly  increase  the  intensity  of  certain  sounds 
by  its  resonance.  For  what  purpose  are  resonators  used? 

What  is  meant  by  the  interference  of  sound  ? How  can  it  be  pos- 
sible for  two  notes  sounded  at  the  same  time  to  produce  silence  ? 

Into  what  three  classes  may  musical  instruments  be  divided? 

Name  some  examples  of  each  class  of  instruments. 

In  what  different  ways  may  the  column  of  air  in  wind  instruments 
be  set  into  vibration  ? By  which  of  these  methods  is  the  air  in  an 
organ-pipe  set  into  vibration?  By  which  in  a melodeon  or  accor- 
dion? 


15 


CHAPTER  III. 

THE  NATURE  OF  HEAT.— THERMOMETERS 
AND  EXPANSION. 

191.  The  Cause  of  Heat. — The  molecules  of  matter 
are  never  at  rest,  but  are  constantly  moving  towards 
and  from  one  another.  Heat  is  caused  by  these  motions. 

In  hot  bodies  the  molecules  are  vibrating  very 
rapidly  through  distances  that  are  great  when  com- 
pared to  tbe  size  of  the  molecules,  wliile  in  cooler 
bodies  they  are  not  moving  so  energetically,  or 
through  as  great  distances. 

The  cause  of  heat  is,  therefore,  similar  to  the  cause  of  sound,  since 
each  is  produced  by  a vibratory  motion  of  matter.  The  nature  of 
this  motion,  however,  is  different.  When  bodies  vibrate  so  as  to  cause 
sound,  the  whole  mass  of  the  body  moves ; when  they  vibrate  so  as  to 
cause  heat,  it  is  only  the  molecules  that  vibrate. 

192.  The  Luminiferous  Ether. — The  bell  shown 
in  Fig.  vd  cannot  be  heard  if  struck  when  the  vessel,  A, 
is  emptied  of  all  the  air  rvhich  filled  it,  since  then  there 
is  nothing  around  the  sounding  bell  which  it  can  set 
into  waves.  But  if  we  hang  a hot  body  in  ichat  seems 
to  us  to  he  an  empty  sjoace,  we  find  that  the  heat  readily 
yiosses  through  this  space.  For  this  and  other  reasons 
we  believe  that  all  space,  even  though  it  appears  to  be 
empty,  is  filled  with  something  which  is  set  into  wave- 

170 


THE  NATURE  OF  HEAT. 


171 


motion  by  tbe  vibrations  of  the  molecules  of  heated 
bodies.  This  something  which  fills  all  space  is  called 
the  luminiferous  ether.^  and  it  is  by  vibrations.,  or  waves 
in  it,  that  heat  is  transmitted  f rom  one  place  to  another. 
It  is  called  the  luminiferous  ether  because  it  also  trans- 
mits light  by  its  vibrations. 

The  luminiferous  ether  fills  all  space,  even  that 
between  the  molecules  of  matter.  The  molecules  of 
hot  bodies,  by  their  shahinys,  cause  waves  in  this  ether, 
just  as  the  shakings  of  the  sides  of  a bell  cause  waves 
in  air,  and  when  these  ether-waves  strike  us,  we  have 
the  sensation  of  heat. 

Since  the  ether  can  pass  through  the  spaces  between  the  molecules, 
and  as  matter  is  porous,  it  is  evident  that  v/e  cannot  mate  a vessel 
ether-tight,  that  is,  wo  cannot  prevent  the  ether  from  passing  into  or 
out  of  it. 

193.  General  Effect  of  Heat. — Although  we  can- 
not see  the  shakings  of  the  molecules  of  a body,  yet 
we  can  see  the  efi:ect  which  these  shakings  produce. 
The  hotter  the  body  gets,  the  more  energetic  becomes 
the  motion  of  the  molecules  ; and  in  this  way  the  force 
of  molecular  attraction  is  partly  overcome,  and  the 
body  expands.  When  a body  loses  heat,  the  molecular 
motion  becomes  less  energetic,  attraction  again  draws 
the  molecules  together,  and  the  body  contracts. 

Whe7i,  therefore,  matter  is  heated,  it  expands,  or  grows 
larger,  because  the  molecules  are  made  to  swing  through 
greater  distances. 

194:.  Temperature.  — When  two  bodies  can  be 
placed  in  contact  without  any  heat  passing  from  one  to 
the  other,  they  are  said  to  be  at  the  same  temperature. 
When  heat  passes  from  one  to  the  other,  the  body 
which  gives  the  more  heat  is  said  to  be  at  the  higher 
temperature. 


172 


NATURAL  PUILOSOPHY. 


The  temperature  of  a body  is  measured  by  the  ther- 
mometer. 

195.  Thermometers. — The  thermometer  depends  for 
^ its  operation  on  the  expansion  of  a liquid 

contained  in  a glass  tube.  The  liquid 
most  commonly  employed  is  mercury.^ 
though  alcohol  is  sometimes  used. 

The  thermometer  consists  of  a long, 
straight  tube,  A i?.  Fig.  85,  of  small  inter- 
nal diameter,  closed  at  the  upper  end,  and 
widened  at  the  lower  end  into  a bulb,  C. 
The  bulb,  (7,  and  part  of  the  tube  con- 
tains  mercury ; the  space  above  the  mer- 
cury in  the  tube  is  a vacuum.  ^Vhen 
the  thermometer  is  taken  into  a hot 
place,  the  mercury,  becoming  warmer, 
expands  and  its  level  rises ; but  when 
c taken  into  a cold  place,  it  loses  heat,  con- 
I®'  tracts,  and  its  level  falls.  Equal  spaces, 
called  degrees,  are  marked  on  the  tube. 

196.  Construction  of  a Thermometer. — While  the 
tube  is  open  at  the  top,  the  bulb  and  part  of  the  tube 
are  filled  with  cold  mercury.  The  bulb  is  then  cau- 
tiously heated.  As  the  mercury  grows  hotter,  it  ex- 
pands, and,  filling  the  tube,  drives  out  all  the  air,  and 
begins  to  flow  over  the  top.  The  heat  is  now  taken 
away  from  the  bulb,  and  at  the  same  time  the  upper 
eiid  is  completely  closed,  by  being  melted  in  the  flame 
of  a blow-pipe.  As  the  bulb  cools,  the  mercury  falls 
in  the  tube,  leaving  a vacuum  or  empty  space  above  it. 

The  tube  is  next  yraduated^  or  divided  into  degrees. 
For  this  purpose,  the  bulb  is  dipped  into  melting  ice, 
and  a mark  made  at  the  point  on  the  tube  to  which  the 


212° 


32° 


100° 


0° 


Fig.  84.  Fig. 
Fahren-  Cen 
heit.  grai 


THE  NATURE  OF  HEAT. 


173 


mercury  sinks.  Tlie  bulb  is  then  exposed  to  the  steam 
rising  from  boiling  water,  and  another  mark  made  at 
the  point  to  which  the  mercury  rises.  These  two 
points  are  called  respectively  the  freezing  and  the  boil- 
ing points.  The  length  of  the  tube  betv:een  these  two 
points  is  then  divided  into  a certain  number  of  equal 
parts  called  degrees.^  and  the  rest  of  the  tube,  that  is, 
the  part  below  the  freezing  point  and  above  the  boil- 
ing point,  is  divided  into  degrees  of  the  same  length. 

There  are  two  thermometer  scales  in  common  use, 
viz.,  the  Fahrenheit  and  the  Centigrade.  These  two 
scales  are  shown  in  Figs.  Si,  85,  In  the  Fahrenheit 
scale,  Fig.  ST,  the  length  of  the  tube  between  the 
freezing  and  boiling  points  is  divided  into  180  equal 
parts  called  degrees.  The  freezing  point  is  marked 
32°,  and  the  boiling  point  212°. 

In  the  Centigrade  scale,  Fig.  85,  the  distance  between 
the  freezing  and  boiling  points  is  divided  into  100 
equal  parts.  In  this  scale  the  freezing  point  of  water 
is  marked  0°,  or  zero,  and  the  boiling  point  100°.  De- 
grees of  the  Fahrenheit  scale  are  indicated  by  an  F. 
or  Fah. ; those  of  the  Centigrade  scale  by  a C.  Thus, 
the  freezing  point  of  water  is  32°  F,  or  0°  C. 


197.  Uses  of  Thermometers. — We  cannot  entirely 
rely  on  our  sensations  to  determine  the  difference 
between  the  temperature  of  two  bodies.  If,  Avhen 
the  hand  is  very  warm,  Ave  plunge  it  into  a basin  of 
tepid  AVciter,  the  AAmter  Avill  feel  cool ; but  if  the  hand 
is  quite  cold  Avhen  Ave  plunge  it  into  the  same  tepid 
Avater,  the  Avater  Avill  feel  warm.  Again,  if  in  Avinter 
we  come  from  the  cold  air  outside  into  the  entry  or 
hall,  the  entry  or  hall  feels  Avarm ; but  if  Ave  go  into 
the  entry  from  the  Avarmer  parlor,  the  entry  then 
15* 


174 


NATURAL  PBILOSOPUY. 


feels  cold,  Tlie  indications  of  a thermometer  are  not 
open  to  these  objections. 

So,  also,  in  a room  where  there  is  no  fire,  and  all 
things  are  at  the  same  temperature,  the  marble  mantle 
feels  cold  to  the  hand,  while  the  hearth-rug  feels  warm. 

W e see,  also,  from  these  remarks,  that  the  words  hot 
and  cold  are  but  relative,  since  the  same  body  may  he 
hot  when  compared  with  one  body,  and  cold  when  com- 
pared with  another.  j 

198.  Expansion  of  Solids.  — Solids  expand  le.'JS  j 
than  liquids,  and  liquids  less  than  gases.  Different  i 
solids  expand  differently  in  amount  when  heated  one  | 
degree.  Zinc,  lead,  and  tin  are  among  the  most  ex-  j 
pausible  of  the  metals,  and  steel  and  platinum  among  ' 
the  least.  Ice  is  more  expansible  than  zinc.  Most  ^ 
crystalline  solids  expand  more  in  one  direction  than  I 
in  another ; while  most  bodies  that  are  not  crystallized, 
expand  equally  in  all  directions. 

199.  Examples  of  the  Expansion  of  Solids. — 

When  solid  bodies  expand  or  contract,  they  exert  con- 
siderable force.  The  tires  of  wheels  are  made  of  such  , 
a size  that,  Avhen  cold,  they  will  not  go  on  the  wheel.  ' 
They  are  heated  until  they  are  large  enough  to  slip  on 
easily,  and  when  cold  they  contract  and  fit  very  tightly 
to  the  Avheel,  and  firmly  hold  its  parts  together. 

The  rails  of  railways  are  laid  with  some  little  dis- 
tance between  their  ends,  so  as  to  leave  room  for  ex- 
pansion ; but  for  these  spaces,  the  force  of  expansion 
would  twist  the  rails  from  the  Avooden  ties. 

The  snapping  and  crackling  of  a stOAm  when  sud- 
denly heated,  as  by  building  a fire,  or  cooled  as  by 
opening  the  door,  is  caused  bj'  the  unequal  expansion 
of  the  metal  at  different  parts. 


THE  NATURE  OF  HEAT. 


175 


Thick  glass-ware  is  apt  to  break  when  suddenly 
heated  or  cooled,  because  the  unequal  expansion  or 
contraction  overcomes  the  cohesion  of  the  particles. 
Suppose,  for  example,  hot  water  be  poured  into  a thick 
glass  tumbler,  then  the  inside  of  the  glass  expanding- 
before  the  outside  becomes  warm,  is  very  apt  to  break 
the  tumbler. 

Experiment. — A glass  vessel  can  be  cut  into  any  desired  shape  as  fol- 
lows ; Suppose  it  is  desired  to  cut  off  the  top  of  a broken  bottle.  Start 
a crack  in  the  edge  of  the  bottle  by  heating  and  suddenly  cooling  it. 
From  the  end  of  this  crack,  draw  a chalk  line  in  the  direction  in  which 
it  is  desired  to  cut  the  bottle.  Heat  the  end  of  a thin  poker  red  hot, 
and  hold  the  heated  point  firmly  against  the  glass,  on  the  chalked 
line,  and  a little  distance  from  the  crack.  In  a moment  a click  will  be 
heard,  and  the  crack  will  extend  to  the  point  of  the  poker.  Lift  the 
poker  from  this  point,  and  place  it  again  on  the  chalked  line  a little 
beyond  the  crack,  and  so  on  until  the  crack  has  extended  all  round. 

Caution. — When  the  poker  has  become  too  cold,  reheat.  With  some 
little  practice,  broken  glass  vessels  can  be  given  a variety  of  useful 
shapes  by  this  method. 

200.  Expansion  of  Liquids. — Most  liquids  expand 
when  beated  and  contract  when  cooled,  and  this  is  true 
whatever  the  temperature. 

Water,  however,  presents  some  curious  exceptions 
to  this  general  statement.  When  water  which  is  at 
the  temperature  of  melting  ice,  or  32°  Fah.,  is  heated, 
it  contracts  and  gets  denser,  and  this  continues  until  it 
reaches  the  temperature  of  39°.2  Fah.;  when  heated 
above  this  point,  however,  it  expands  like  most  other 
substances.  The  temperature  of  39°.2  Fah.  is  called 
the  temperature  of  the  maximum  or  greatest  density 
of  water.  When  water  is  at  the  temperature  of  its 
maximum  density,  it  will  expand  whether  it  be  heated 
or  cooled.  The  general  effect  produced  in  all  matter 
by  an  increase  of  temperature  is  to  cause  it  to  expand, 
while  a decrease  of  temperature  causes  it  to  contract. 


176 


NATURAL  PHILOSOPHY. 


Experiment.  — Fit  a narrow  glass  tube,  A,  Fig.  86,  tightly  into  a 
cork,  B,  and,  inserting  the  cork  into  the 
neck  of  a large  bottle,  C,  fill  the  bottle 
and  tube  with  water  to  the  point  A. 
Stand  the  bottle  in  a pan,  D,  and  sur- 
round it  by  layers  of  broken  ice  and 
salt,  or  snow  and  salt.  As  the  water  is 
cooled,  the  column  of  water  in  the  tube 
will  fall,  thus  proving  that  the  water  is 
contracting.  But  when  the  water  is 
cooled  to  39°. 2 Fah.,  the  column  will 
cease  falling,  and  will  begin  to  rise,  al- 
though the  water  is  still  growing  colder. 

Caution. — This  experiment  will  be  more 
successful,  the  larger  the  bottle  and  the 
Fig.  86.— Expansion  of  Water  smaller  the  diameter  of  the  tube.  To 
enable  those  at  a distance  to  see  the  water 
in  the  tube,  it  may  be  slightly  colored  with  bluing. 


201.  Effect  on  the  Freezing  of  Large  Bodies  of 
Water.  — Let  A,  Fig.  87,  be  a deep  lake  of  fresh  water, 
in  which  the  water  throughout  the  whole  lake  is  at 


Fig.  87.— Effect  of  Maximnin 
Density  on  Freezing. 


the  temperature  of  50°  Fah. 
If,  now,  the  air  grows  cold, 
the  water  at  the  surface  of 
the  lake  grows  cold  and  falls, 
because  it  becomes  heavier 
than  the  other  water.  When, 
however,  all  the  water  in  the  lake  reaches  the  tempera- 
ture of  39°.2  Fah.,  it  is  as  heavy  as  it  can  yet  by  loss  of 
heat.  If  the  water  near  the  surface  now  continues  to 
lose  its  heat,  it  will  grow  lighter,  and  will  remain  at 
the  surface  until  changed  into  ice.  Since  water  con- 
ducts heat  poorly,  the  water  at  the  bottom  remains 
unfrozen.  Were  it  not  for  this  peculiarity  in  its  ex- 
pansion, the  water  throughout  the  entire  lake  would 
continue  to  cool  until  the  Avhole  mass  became  solid, 
Avhen  not  even  a summer’s  heat  would  entirely  melt  it. 


THE  NATURE  OF  HEAT. 


177 


202.  Expansion  of  Gases.  — When  our  atmos- 
phere is  lieated,  it  expands ; when  cooled,  it  contracts. 
When  any  mass  of  air  is  heated  it  expands,  and,  be- 
coming lighter  than  the  surrounding  air,  rises,  while 
the  cooler  air  on  the  sides  blows  in  towards  the  place 
from  which  the  heated  air  has  risen.  Winds  are  caused 
in  this  way. 

The  draught  in  a chimney  is  caused  b}'-  the  air 
within  the  chimney  being  heated  and  rising,  and  the 
cooler  air  rushing  in  through  the  fire  to 
take  the  place  left  by  the  rising  air. 

Experiment. — A large  bottle,  A,  of  thin  glass,  is  pro- 
vided with  a narrow  glass  tube  fitted  to  a cork,  and 
inserted  in  the  neck  of  the  bottle.  Hold  the  bottle  for 
a few  minutes  near  a gas  flame  or  hot  stove,  and  then 
quickly  place  it,  as  shown  in  Fig.  88,  with  the  end  of 
the  tube  dipping  below  the  surface  of  some  colored  liquid 
placed  in  a vessel,  £.  As  the  air  in  the  bottle  cools,  it 
will  contract,  and  the  pressure  of  the  outside  air  will 
cause  a column  of  water  to  mount  in  the  tube.  If,  how- 
ever, the  air  in  the  bottle  be  again  heated,  as  by  holding 
the  hand  against  it,  the  expansion  of  the  air  will  drive 
the  column  of  colored  liquid  down.  This  apparatus  will, 
therefore,  act  like  a thermometer ; only  the  column  of 
colored  liquid  falls  when  the  air  is  warm,  and  rises  when 
it  is  cold.  It  also  in  part  acts  like  a barometer. 

Caution.  — Do  not  hold  the  bottle  too  long  by  the 
fire,  or  else,  when  placed  in  B,  the  liquid  will  rise  and  completely  fill 
the  tube. 


Fig.  88.  — Ex- 
pansion and 
Contraction 
of  Air. 


Syllabus. 

Heat  is  caused  by  the  vibrations  of  the  molecules  of  matter.  Heat 
is  transmitted  by  means  of  waves  produced  by  the  vibrations  of  the 
molecules  in  a medium  called  the  luminiferous  ether.  This  medium  is 
believed  to  fill  all  space,  and  even  to  penetrate  the  invisible  pores  be- 
tween the  molecules. 


M 


178 


NATURAL  PHILOSOPHY. 


We  cannot  make  a vessel  ether-tight  because  we  do  not  know  of 
any  substance  whose  molecules  have  no  spaces  between  them. 

The  general  effect  of  heat  upon  matter  is  to  cause  it  to  expand.  It 
expands  because, when  heated,  the  molecules  vibrate  through  greater 
distances. 

We  measure  temperature  by  means  of  an  instrument  called  a ther- 
mometer. The  thermometer  depends  for  its  operation  on  the  expan- 
sion of  a liquid  contained  in  a tube. 

The  space  above  the  mercury  in  a thermometer  is  vacuous.  The 
tube  is  graduated  into  degrees  by  dividing  the  length  between  the 
freezing  and  boiling  points  into  a given  number  of  parts.  In  the  Fah- 
renheit scale  this  length  is  divided  into  180  parts,  and  in  the  Centi- 
grade scale  into  100  parts. 

We  cannot  entirely  rely  on  our  sensations  to  distinguish  between 
hot  and  cold  bodies,  since  bodies  at  the  same  temperature  may  some- 
times feel  hot  and  sometimes  cold. 

Solid  bodies  exert  considerable  force  in  expanding  and  contracting. 
Tires  are  shrunk  firmly  on  wheels  by  heating  the  tires  until  they  are 
large  enough  to  slip  on  the  wheels,  and  then  cooling. 

When  water  at  the  temperature  of  32^  Fab.  is  heated,  it  contracts  until 
it  reaches  the  temperature  of  39.2°  Fah.,  which  is  called  the  temperature 
of  its  greatest  density.  This  peculiarity  of  water  prevents  large  bodies 
of  fresh  water  from  becoming  frozen  throughout. 

Winds  are  caused  by  the  expansion  of  air  by  heat.  When  any 
mass  of  air  becomes  heated,  it  expands,  grows  lighter  and  rises,  while 
the  cooler  air  blows  in  from  all  sides. 

The  draught  of  a chimney  is  caused  by  the  expansion  of  the  air 
within  the  flue. 

A simple  thermometer  may  be  made  by  fitting  a tube  tightly  in  the 
neck  of  a large  bottle,  gently  heating  the  bottle,  and  then  dipping  the 
end  of  the  tube  into  a vessel  filled  with  water  or  colored  liquid. 


Questions  for  Review. 

What  is  the  cause  of  heat?  By  what  means  is  heat  transmitted 
from  one  place  to  another  ? How  do  the  vibrations  which  cause  heat 
differ  from  those  which  cause  sound  ? 

What  is  meant  by  the  luminiferous  ether  ? Why  do  we  think  that 
it  exists  ? 

Define  expansion.  Why  should  bodies  expand  when  they  are  heated  ? 


QUESTIONS  FOR  REVIEW. 


179 


When  are  two  bodies  said  to  be  at  the  same  temperature?  How  is 
temperature  measured  ? 

Describe  the  construction  of  a thermometer.  What  method  is  gen- 
erally adopted  for  filling  the  bulb  ? How  are  the  freezing  and  boiling 
points  obtained  ? How  is  the  length  of  a degree  for  any  thermometer 
determined?  Explain  the  difference  between  the  Fahrenheit  and  the 
Centigrade  scale. 

Why  can  we  not  entirely  rely  on  our  sensations  to  determine  the 
difference  of  temperature  between  any  two  bodies?  Show  that  hot 
and  cold  are  relative  terms. 

Name  some  of  the  most  expansible  metals ; name  some  of  the  least 
expansible.  Give  any  instances  of  the  practical  application  of  the 
force  exerted  by  the  expansion  or  contraction  of  solids. 

How  may  a broken  glass  vessel  be  cut  into  any  desired  shape? 

Name  any  example  of  the  expansion  of  a liquid.  What  exception  to 
the  general  statement  that  bodies  expand  by  heat  does  water  present? 
What  effect  has  this  fact  on  the  freezing  of  large  bodies  of  water  ? Why  ? 

Describe  an  experiment  by  which  it  can  be  shown  that  water  some- 
times contracts  on  being  heated. 

Explain  the  cause  of  winds.  Why  does  air  rise  when  it  is  heated? 

What  is  the  cause  of  draught  in  chimneys?  Whj’  is  the  draught 
stronger  when  the  fire  is  hotter  ? 

Describe  in  full  the  construction  of  a simple  thermometer  from  a 
bottle,  tube,  and  vessel  of  liquid.  What  causes  the  liquid  to  fall  in 
the  tube  ? What  causes  it  to  rise  ? 


CHAPTER  IV. 

THE  COMMUNICATION  OF  HEAT.  — THE 
SURFACE  ACTION  OF  BODIES. 

203.  The  Communication  of  Heat.  — Heat  may  i 
be  communicated  or  transferred  from  one  body  to 
another  in  three  ways,  viz.,  hy  conduction^  by  convec- 
tion^ and  hy  radiation.  Heat  is  mainly  communicated 
or  distributed  through  solids  by  conduction ; through 
liquids  and  gases  mainly  by  convection  and  radiation. 

204:.  Conduction  of  Heat.  — If  one  end  of  a bar  | 
of  iron,  copper,  or  any  other  metal,  be  placed  in  a fire,  j 
the  other  end  will  after  awhile  become  too  hot  to  be  \ 
held.  The  heat  of  the  fire  has  been  communicated  to  i 
the  rod,  and  carried  through  it  by  conduction. 

When  the  molecules  at  the  heated  end  of  the  bar 
are  set  into  motion  by  the  heat  of  the  fire,  they  grad- 
ually impart  their  motion  to  the  molecules  beyond 
them,  and  in  this  way  the  heat  is  conducted  through 
the  body.  The  rapidity  of  the  conduction  from  one 
end  of  a bar  to  the  other  is  the  same  whether  the  bar 
be  straight  or  bent. 

I 

205.  Differences  of  Conductivity.  — The  rapidity  j 
with  which  heat  is  conducted  varies  greatly  in  differ-  j 
ent  solids.  If  two  rods  of  the  same  length  and  thick-  i 
ness,  one  of  copper  and  the  other  of  iron,  be  put  to- 

180 


THE  COMMUNICATION  OF  NEAT. 


181 


Fig,  89. — Unequal  Conduction  of  Copper  and  Iron. 


gether,  witli  one  end  of  each  in  the  fire,  the  heat  will 
be  conducted  through  the  copper  rod  much  more 
rapidly  than  through  the  iron  one.  Again,  if  a rod 
of  glass  be  used  in  the  same  way,  it  will  be  found  to 
be  a very  bad  conductor  of  heat. 

Experiment. — Get  two  bars  — A,  of  copper,  and  B,  of  iron  — of 
the  same  length  and  thick- 
ness. By  means  of  wax, 
stick  a number  of  marbles 
or  buck-shot  to  each  at 
equal  distances,  as  shown 
in  Fig.  89.  Now  expose 
one  end  of  each  bar  to  the 
same  source  of  heat,  as, 
for  example,  the  flame  of  an  alcohol-lamp,  and  it  will  be  observed 
that  more  balls  will  fall  in  the  same  time  from  the  copper  bar  than 
from  the  iron,  thus  showing  that  it  is  the  better  conductor  of  heat. 

Experiment — An  alcohol-lamp  can  be  easily  made  as  follows  : Bore 
a hole  through  a fine-grained  cork,  and  insert  in  it  a small  piece  of 
glass  tubing,  or  a bit  of  tin  rolled  in  the  shape  of 
a tube,  and  through  this  pass  a piece  of  wick. 

Place  the  cork  and  wick  in  a bottle  filled  with  al- 
cohol. 

Experiment. — Where  illuminating  gas  is  at  hand, 
a more  convenient  source  of  heat  is  furnished  by 
the  Bunsen  burner,  one  of  which  can  be  easily  made 
as  follows : Take  a piece  of  tin  in  the  shape  of  a 
rectangle,  and  cut  out  small  pieces  from  one  of  its 
shorter  ends,  so  that,  when  the  tin  is  rolled  in  the 
form  of  a hollow  tube,  as  shown  at  B,  Fig.  90,  there 
will  be  openings  provided  as  at  A.  Fit  the  end, 

C,  loosely  over  a gas-burner.  Turn  on  the  gas,  and 
light  it  from  above.  If  the  burner  has  been  properly 
made,  the  flame  will  be  bluish  and  almost  non-lumi- 
nous,  and  will  not  soot  articles  heated  in  it.  Air 
enters  at  the  openings  below,  and  burns  the  gas 
more  thoroughly  than  an  ordinary  gas-burner.  This 
flame  is  very  hot;  glass  tubes  held  in  it  may  be 
softened  and  bent  in  any  desired  shape. 

Caution.  — If  the  burner  is  not  well  made,  the  gas  will  often  ignite 
at  the  gas-burner,  and  burn  with  a luminous  flame.  The  fault  will 
16 


Fig.  90.— A Simple 
Bunsen  Burner. 


182 


NATURAL  PHILOSOPHY. 


generally  be  found  to  be  due  either  to  the  holes  below  not  being  large 
enough,  or  the  tin  tube  not  being  of  sufficient  length. 

206.  Applications  of  the  Conductivity  of  Solids. — 

When  a hot  body  is  placed  in  contact  with  a cold 
body  whose  conducting  power  is  good,  or  if  a cold 
body  be  placed  on  a hot  conductor,  the  cold  body  may 
become  of  the  same  temperature  as  the  hot  body.  If 
we  wish  to  keep  a body  at  the  same  temperature  for 
any  time,  we  must  surround  it  by  some  substance  that 
conducts  heat  very  ^worly. 

Ice  is  wrapped  in  blankets  to  keep  from  melting, 
because  the  blankets  are  poor  conductors  of  heat,  and 
therefore  prevent  the  heat  from  entering. 

We  wear  thick  woollen  clothes  in  winter  to  keep 
the  heat  of  our  bodies  from  escaping  rapidly.  Thin 
muslin  or  linen  clothes  are  comfortable  in  summer 
because  they  readily  permit  the  heat  of  the  body  to 
escape. 

Ice-hoQses  are  made  with  thick,  double  walls,  filled 
with  shavings  or  saw-dust,  to  keep  the  outside  heat 
from  entering.  Fire-proof  safes  have  double  or  triple 
walls  filled  with  some  poor  conductor  for  the  same 
purpose. 

The  interior  of  the  earth  is  believed  to  be  very  hot; 
very  little  of  the  heat,  however,  reaches  the  surface, 
owing  to  the  low  conducting  power  of  the  materials 
of  the  crust  of  the  earth. 

207.  Conduction  of  Fluids.  — Liquids  and  gases 
are  very  bad  conductors  of  heat.  When  heated  from 
below,  they  grow  hot  by  a process  called  convection; 
but  when  heated  from  above,  they  conduct  but  very 
little  heat  downwards.  The  poor  conducting  power 
of  water  may  be  shown  by  the  following 


THE  COMMUNICATION  OF  HEAT. 


183 


Experiment. — Insert  the  tube  of  the  simple  thermometer,  described 
in  a previous  experiment,  in  a cork,  and  place  it  in 
the  small  end,  A.  of  a funnel,  the  neck  of  which  has 
been  cut  off.  Fill  the  funnel  with  water,  so  as  to 
cover  the  bulb,  B,  to  the  depth  of  about  one-fourth 
of  an  inch,  as  shown  in  Fig.  91.  Pour  common 
ether  on  the  water  and  set  fire  to  it,  and  the  heat, 
although  rather  intense,  will  scarcely  be  conducted 
downwards  through  the  water  enough  to  cause  the 
level  of  the  column  of  colored  water  at  h to  fall. 

Caution. — Select  a funnel  of  as  thin  glass  as  pos- 
sible, so  as  to  avoid  its  being  broken  by  the  heat. 

208.  Convection,  — • Wlien  a vessel 
containing  a liquid  is  placed  over  a 
source  of  heat,  the  liquid  at  the  bottom  pig.gi.  — vTater 
of  the  vessel,  by  touching  the  hot  bot- 
tom, is  heated,  and  expanding  becomes 
lighter  and  riseS)  its  place  being  filled  by  some  of  the 
cooler  portions  of  liquid  falling.  These  in  turn  be- 
come heated,  and  are  replaced  by  other  portions,  until 
at  last  the  whole  mass  of  the  liquid  is  at  the  same 
temperature  throughout.  Gases  are  heated  in  the 


Poor  Conductor 
of  Heat. 


same  way. 

Liquids  and  gases,  therefore,  when  heated  are  so 
stirred  about  by  the  heat,  that  all  parts  are  brought 
into  contact  with  the  sides  of  the  vessel.  This  method 
of  communicating  heat  is  called  convection.  Liquids  are 
cooled  in  a similar  manner. 

Experiment.  — Place  a lump  of  ice  in  a tumbler  of  tepid  water. 
As  the  water  which  touches  the  ice  is  cooled,  it  be- 
comes heavier,  and,  sinking,  pushes  up  the  warmer 
and  lighter  water  from  below.  By  watching  the 
water  closely,  these  currents  can  be  seen,  especially 
if  there  are  any  little  specks  of  dirt  in  the  water. 

Their  general  direction  is  shown  in  Fig.  92  by  the 
arrows. 

During  convection,  the  warmer  por- 


Fig.  92. — Convec- 
tion Currents. 


184 


NATURAL  PHILOSOPHY. 


tions  of  tlie  liquid  or  gas  always  move  towards  the 
cooler  portions,  and  tlie  cooler  portions  towards  the 
warmer  portions. 

Winds  are  huge  convection  currents  caused  by  the 
unequal  heating  of  different  portions  of  the  earth. 
The  constant  currents  which  occur  in  the  ocean  are 
convection  currents  in  the  water,  and  are  caused  by  1 
differences  of  temperature  between  the  equator  and  j 

the  poles.  I 

1 

209.  The  Radiation  of  Heat.  — If  a hot  body  be  | 
held  above  the  hand,  its  heat  will  be  felt  by  the  hand. 
Now,  since  gases  are  very  poor  conductors  of  heat,  ; 
this  heat  cannot  have  been  communicated  to  the  hand 
by  conduction  ; nor  can  the  hand  have  received  it  by 
convection,  since  the  hot  currents  of  air  rise.  It  must,  , 
therefore,  have  been  communicated  in  some  other 
way. 

The  cause  of  heat,  as  we  have  seen,  is  the  vibra- 
tions of  the  molecules  of  the  heated  bod}-.  These 
molecules  are  surrounded  by  the  luminiferous  ether, 
and  by  their  motion  cause  waves  in  this  ether.  Heat 
communicated  m this  tvay,  hy  the  luviinif erous  ether., 
is  called  radiant  heat,  and  the  process  is  called  radia- 
tion. 

The  heat  of  the  sun  crosses  the  otherwise  empty 
space  between  the  earth  and  the  sun,  by  means  of 
waves  in  the  luminiferous  ether,  that  is,  hy  radiation. 
When  these  waves  strike  against  a body,  if  they 
cause  its  molecules  to  vibrate,  the  body  is  thus  made 
warm. 

210.  Heat  Radiated  in  all  Directions. — If  we  stand 
at  the  same  distance  from  a stove,  but  on  different 
sides,  we  will  find,  if  the  stove-door  is  shut  and  the 


THE  COMMUNICATION  OF  HEAT. 


185 


i stove  is  of  the  same  material  all  around,  that  the  in- 
i tensity  of  the  heat  thrown  out  or  radiated  is  the  same 
i in  all  directions.  If  a tin  vessel  be  filled  with  boiling 
1 water,  a delicate  thermometer  held  anywhere  at  the 
I same  distance  from  the  sides  will  receive  the  same 
1 amount  of  heat. 

j The  motion  of  the  molecules  of  heated  surfaces  is 
! communicated  through  the  ether  equally  well  in  all 
; directions.  Heated  bodies^  therefore^  radiate  their  heat 
I upwards^  downwards,  or  in  any  direction  equally  well. 

211.  Heat  Radiated  in  Straight  Lines.  — If  the 

medium  through  which  the  heat  is  passing  be  uniform 
throughout,  the  heat  which  is  radiated  from  a body 
passes  off  from  it  in  straight  lines.  If  a screen,  through 
which  heat  cannot  pass,  be  interposed  between  a ther- 
' mometer  and  a source  of  heat,  so  that  any  straight  line 
drawn  from  the  source  of  heat  to  the  edge  of  the 
screen  will  just  pass  the  bulb  of  the  thermometer,  the 
thermometer  will,  by  its  indications,  show  that  it  is 
no  longer  receiving  heat  from  the  source.  If,  how- 
; ever,  the  screen,  be  lowered,  the  thermometer  will 
j rise  in  temperature.  On  hot  days  it  is  cooler  in  the 
shade  than  in  the  sunshine,  because  the  heat  which 
accompanies  the  light  moves,  like  it,  in  straight  lines. 
A single  line  of  heat  is  called  a ray. 

When  a ray  of  heat  passes  from  one  medium  to 
another  of  different  density,  it  is  bent  out  of  its 
straight  course,  or  is  refracted  as  with  a ray  of  sound. 

A common  burning-glass  owes  its  power  of  heating 
things  placed  at  a certain  distance  in  front  of  it,  be- 
cause it  is  so  shaped  that  the  rays  of  heat  from  the 
sun  are  so  bent  by  refraction  on  passing  through  it 
16* 


186 


NATURAL  PHILOSOPHY. 


that  when  they  pass  out  they  all  come  together  at 
nearly  one  point. 

212.  Intensity  of  Radiant  Heat.  — The  hotter  a I 
body,  the  more  intense  the  heat  radiated  from  it.  As  I 
the  molecules  of  a hot  body  have  a greater  amount  of  ! 
motion  than  those  of  a cooler  body,  the  motion  the  : 
hot  body  communicates  to  the  surrounding  ether  must  , 
be  greater  than  that  communicated  by  the  cooler  body,  i 

The  intensity  of  radiant  heat  is  inversely  propor- 
tional to  the  square  of  the  distance.  The  further  we 
go  from  a hot  body,  the  less  is  the  intensity  of  the 
heat  we  receive.  If  we  are  twice  as  far  from  the  hot 
body  at  one  time  as  at  another,  the  intensity  of  the 
radiant  heat  will  be  four  times  less.  This  decrease  in  i 
the  intensity  of  heat,  with  the  increase  of  the  distance, 
is  similar  to  the  decrease  in  the  intensity  of  sound. 

213.  Luminous  and  Obscure  Heat.  — The  heat 
which  is  radiated  from  a luminous  source  is  called  i 
luminous  heat.,  and  that  radiated  from  a non-luminous 
source  obscure  heat.  Luminous  heat  almost  always 
contains  in  addition  obscure  heat. 

214.  Effects  which  Occur  at  the  Surface  of  Bodies. 

— When  the  ether-waves  strike  against  the  surface 
of  a body,  they  are  either  reflected.^  absorbed,  or  tram-  ■ 
mitted. 

215.  Reflection  of  Heat.  — When  the  ether- waves  ■ 
strike  against  any  suitable  surface,  they  are,  like  the 
waves  in  any  elastic  medium,  reflected  from  it.  The 
reflection  of  heat  is  like  the  reflection  of  sound  — 
the  angle  of  reflection  is  equal  to  the  angle  of  inci- 
dence. 

Bodies  vary  greatly  in  their  power  of  reflecting  heat. 


THE  COMMUNICATION  OF  HEAT. 


187 


I Some,  like  smootli,  polished  silver,  will  reflect  nearly 
all  the  heat  which  falls  upon  them ; while  others,  like 
a rough,  soot-covered  surface,  will  reflect  but  little. 

; As  a rule,  bright  polished  metallic  surfaces  are  good 
reflectors  of  heat. 

Experiment.  — Hold  a brightly  polished  flat  piece  of  tin  in  front  of 
an  open  fire,  so  as  to  catch  the  light  and  heat.  Notice  where  the 
patch  of  light  reflected  from  the  tin  falls.  This  spot  will  be  found  to 
be  warmer  than  the  space  around  it,  thus  showing  that  the  heat  of  the 
fire  is  reflected  as  well  as  the  light.  Hold  the  tin  so  that  the  light  of 
the  fire  is  thrown  into  the  face,  and  an  increase  of  temperature  will 
be  felt. 


216.  Absorption  of  Heat. — When  the  ether-waves 
! strike  against  the  surface  of  a body,  those  which  are 
not  reflected  pass  into  the  body.  If,  in  passing  through 
the  body,  the  waves  give  up  their  motion  to  the  mole- 
cules.^ the  body  becomes  hot,  and  we  say  that  the  heat 
is  absorbed',  if,  however,  the  ether-waves  in  passing 
through  the  body  simply  set  in  motion  the  ether  that 
! occupies  the  spaces  between  the  molecules,  Avithout 
moving  the  molecules,  the  radiant  heat  simply  passes 
through  the  body  without  warming  it,  and  Ave  say 
that  the  body  is  diathermanous,  or  transparent  for 
heat. 

When  a body  radiates  heat,  it  gives  the  motion  of 
its  molecules  to  the  ether  outside  it ; Avhen  it  ab- 
sorbs heat,  its  molecules  are  set  into  motion  by  the 
ether-waves  striking  against  them. 

..If  a body  is  a good  absorber  of  heat,  it  must  also  he 
a good  radiator  or  emitter  of  heat.  Lamp-black  is  both 
a good  absorber  and  radiator  of  heat. 

If  a body  is  a good  reflector  of  heat,  that  is,  if  it 
throws  off  most  of  the  ether-Avaves  from  its  surface, 
it  of  course  cannot  be  a good  absorber. 


188 


NATURAL  PniLOSOPHY. 


Good  reflectors  of  heat  are  had  ahsorhers  of  heat,  and 
good  ahsorhers  are  had  reflectors. 

Aiiytliing  which  increases  the  reflecting  power  of  a 
body,  therefore,  must  decrease  its  absorptive  and  radiat- 
ing power,  and  anything  which  increases  the  absorp- 
tive or  radiating  power  must  decrease  the  reflecting 
power.  Thus,  smoothing  and  polishing  diminishes  the 
absorptive  and  radiating,  but  increases  the  reflecting 
power.  The  opposite  effects  are  produced  by  rough- 
ening or  dulling  the  surface. 

217.  Applications  of  the  Absorptive,  Emissive, 
and  Reflecting  Powers.  — Coftee  and  tea  are  brought 
to  the  table  in  brightly  polished  pots,  which,  being  good 
reflectors  of  heat,  are  bad  radiators,  and  the  contents 
of  the  pots  Avill  therefore  keep  Avarm  longer  than  if 
the  outside  Avas  rough  and  dull. 

Meat-roasters  are  so  arranged  that  the  radiant  heat 
of  the  fire  is  reflected  from  the  surfaces  of  brightly 
polished  tin  on  to  the  meat  to  be  cooked. 

If  the  outside  of  a stOA'e  be  too  brightly  polished, 
much  less  heat  Avill  be  radiated  into  the  room  than  if 
the  outside  is  rough  and  dull. 

218.  Selective  Absorption. — Not  only  do  different 
substances  Amry  in  their  poAver  of  absorbing  heat,  but 
even  the  same  body  vmries  in  the  ability  it  possesses 
of  absorbing  the  heat  from  different  sources.  Thus,  a 
surface  covered  Avith  Avhite  lead  will  absorb  nearly  all 
the  heat  radiated  to  it  from  a copper  vessel  filled  Avith 
boiling  water ; Avhile  it  Avill  only  absorb  about  half 
of  the  heat  radiated  from  an  oil-lamp.  In  other  words, 
the  same  body  may  vary  in  its  power  of  ahsorhing  lu- 
minous and  non-luminous  heat. 


TEE  COMMUNICATION  OF  HEAT. 


189 


219.  Cause  of  Selective  Absorption.  — We  have 
! seen  that  sound-waves  striking  against  the  strings  of  a 

piano  may  give  part  of  their  motion  to  the  strings,  pro- 
vided they  can  vibrate  at  the  same  rate  as  the  sonnd- 
I waves  which  strike  them,  or,  in  other  words,  the 
sound-waves  excite  sympathetic  vibrations  in  the 
strings.  The  same  is  true  with  the  ether-waves ; if 
they  strike  against  a body  whose  molecules  are  able 
to  vibrate  at  the  same  rate  that  they  do,  then  the  heat 
: is  absorbed,  since  the  molecules  are  set  into  vibration ; 
otherwise  the  ether-waves  simply  pass  through  the 
body  by  setting  the  ether  between  the  molecules  into 
motion. 

220.  Diathermancy. — When  the  ether-waves  pass 
through  a body  without  heating  it,  that  is,  when  they 
pass  through  without  giving  their  motion  to  its  mole- 
cules, we  say  that  the  body  is  diatliermanous.^  or  trans- 
parent to  heat ; when  it  will  not  let  the  heat  so  pass 
through,  it  is  called  athermanous.^  or  opaque  to  heat. 

Clear  rock-salt  is  very  diathermauous  to  all  kinds 
I of  heat.  Dry  air  is  very  diathermauous,  but  when  it 
contains  water  vapor,  it  is  rendered  less  diathermauous. 

It  is  mainly,  therefore,  to  the  vapor  of  water  which 
our  atmosphere  contains  that  it  owes  its  power  of  ab- 
sorbing a part  of  the  sun’s  heat. 

Diathermancy  is  independent  of  transparency.  Thus, 
alum,  which  freely  allows  light  to  pass  through  it, 
stops  obscure  heat ; while  smoky  quartz,  which  is 
almost  opaque  to  light,  allows  heat  to  pass  through 
it.  A solution  of  iodine  in  bisulphide  of  carbon  is 
opaque  to  light,  but  very  transparent  to  heat. 

Experiment.  — Hold  a piece  of  common  window  glass  between  tlie 
face  and  the  open  door  of  a fire,  and  it  will  be  found  that  the  face 
will  feel  less  hot  than  when  not  shielded  by  the  glass ; the  glass,  there- 


190 


NATURAL  PHILOSOPET. 


fore,  is  atliermanous  to  the  heat  of  the  fire.  Now  hold  the  glass  be- 
tween the  face  and  the  sun,  and  no  perceptible  difference  will  be  felt 
in  the  heat,  whether  the  face  be  shielded  by  the  glass  or  not;  the 
glass,  therefore,  is  diathermanous  to  the  sun’s  heat. 




Syllabus. 

Heat  is  communicated  in  three  ways;  viz.,  by  conduction,  by  con- 
vection, and  by  radiation. 

When  heat  is  communicated  by  conduction,  the  molecules  impart 
their  motion  to  the  molecules  beyond  them,  and  in  this  way  the  heat 
is  conducted  from  one  part  of  the  body  to  anotlier. 

Substances  vary  greatly  in  their  power  of  conducting  heat ; some 
are  excellent  conductors,  while  others  are  very  poor  conductors. 

When  we  wish  to  keep  a body  at  a constant  temperature,  we  gen- 
erally surround  it  with  some  substance  that  is  a poor  conductor. 

Clothes  keep  the  body  warm  by  keeping  the  heat  in.  Blankets 
keep  ice  from  melting  by  keeping  the  heat  out. 

Ice-houses  and  fire-proof  safes  are  made  witli  double  or  triple  hollow 
walls,  filled  with  some  poor  conducting  substance,  so  as  to  preserve 
their  contents  from  the  effects  of  external  heat. 

Gases  and  liquids  are  very  poor  conductors  of  heat,  but  very  little 
heat  being  conducted  downwards  through  them.  They  are  generally 
heated  by  convection. 

When  a liquid  or  a gas  is  heated,  the  difference  of  density  between 
the  hot  and  the  cool  parts  causes  currents  called  convection  currents, 
which  thoroughly  stir  the  liquid  or  gas. 

AVhen  a lump  of  ice  is  put  in  a tumbler  of  water,  the  warm  water 
is  brought  into  contact  with  the  ice  by  means  of  convection  currents. 

When  the  molecules  of  a hot  bod_v  impart  their  motion  to  the  ether 
outside  them,  they  are  said  to  radiate  heat.  Bodies  radiate  or  give 
off  their  heat  equally  in  all  directions. 

The  intensity  of  radiant  heat  is  directly  proportional  to  the  tem- 
perature of  the  body  from  which  it  is  radiated,  and  inversely  propor- 
tional to  the  square  of  the  distance  from  the  radiating  body.  Heat 
is  radiated  in  straight  lines  as  long  as  it  passes  through  a uniform 
medium.  When  it  passes  from  one  medium  to  another  of  different 
density,  it  is  turned  out  of  its  course  or  refracted. 

Bodies  held  at  a certain  distance  from  a burning-glass  are  heated 
because  the  rays  of  the  sun  are  so  refracted  by  passing  through  the 
glass  that  when  they  come  out  they  all  collect  at  nearly  one  point. 


QUESTIONS  FOR  REVIEW. 


191 


Heat  from  a luminous  source  is  called  luminous  heat ; that  from  a 
non-luminous  source,  obscure  heat.  Luminous  heat  is  generally  ac- 
companied by  obscure  heat. 

When  the  ether-waves  strike  against  the  surface  of  a body,  they  are 
either  reflected,  absorbed,  or  transmitted. 

When  heat  is  reflected,  the  angle  of  reflection  is  equal  to  the  angle 
of  incidence.  Polished  metals  are  good  reflectors. 

The  incident  beat  which  is  not  reflected  passes  into  the  body.  If,  in 
passing  through  it,  the  ether-waves  give  their  motion  to  the  molecules 
of  the  body,  the  body  is  warmed ; and  we  say  that  the  heat  is  absorbed. 

If  the  waves  pass  through  a body  without  giving  their  motion  to  the 
molecules,  the  body  remains  cold,  and  the  heat  is  transmitted.  The 
body  is  then  said  to  be  diathermanous. 

Substances  which  possess  high  reflecting  powers  have  but  feeble 
powers  of  radiation  or  absorption,  while  those  which  have  high  ra- 
diating or  absorptive  powers  possess  but  feeble  reflecting  powers. 

The  absorptive  and  diathermanic  powers  of  the  same  substance 
vary  with  different  kinds  of  heat.  Many  substances  which  will  ab- 
sorb or  transmit  obscure  heat  very  well,  absorb  or  transmit  luminous 
heat  either  very  slightly  or  not  at  all. 

Clear  rock-salt  is  diathermanous,  or  transparent  to  all  kinds  of  heat. 
A solution  of  iodine  in  bisulphide  of  carbon  is  opaque  to  luminous 
heat,  but  transparent  to  obscure  heat. 

Questions  for  Review. 

In  what  three  ways  may  heat  be  communicated  ? By  which  of 
these  is  heat  communicated  in  solids?  By  w'hich  are  liquids  and  gases 
heated  ? 

Name  some  good  conductors  of  heat ; name  some  poor  conductors. 
How  may  poor  conductors  be  employed  to  keep  the  temperature  of  a 
body  uniform? 

Describe  any  experiment  showing  the  difference  in  the  conducting 
power  of  different  substances.  Explain  how  a simple  Bunsen  burner 
may  be  constructed. 

Are  liquids  good  conductors  of  heat  ? How  can  this  be  shown  ex- 
perimentally ? 

Define  convection.  By  what  are  convection  currents  caused?  Give 
an  example  of  huge  convection  currents  in  air  and  in  water. 

What  is  meant  by  the  radiation  of  heat?  By  what  means  do  bodies 
radiate  their  heat  ? How  does  the  sun’s  heat  reach  the  earth  ? 


192 


NATURAL  PHILOSOPHY. 


Is  heat  radiated  any  better  in  one  direction  than  in  another  ? Upon 
what  does  the  intensity  of  radiant  heat  depend  ? 

How  is  the  intensity  of  heat  affected  by  an  increased  distance  from  ; 
the  body  radiating  the  heat  ? Is  heat  radiated  in  straight  or  curved  I 
lines  ? How  can  you  prove  this  ? I 

Distinguish  between  luminous  and  obscure  heat.  Define  reflection  ' 
of  heat.  Is  the  reflection  of  heat  similar  to  the  reflection  of  sound?  \ 
When  are  the  ether-waves  which  strike  the  surface  of  a body  ab-  1 
sorbed?  When  are  they  transmitted?  ^ 

What  name  is  given  to  a substance  which  transmits  heat?  What  ' 
is  it  called  when  it  does  not  transmit  heat? 

Why  are  good  reflectors  of  heat  poor  radiators  and  absorbers  ? Why 
are  good  radiators  and  absorbers  poor  reflectors  ? 

Will  coffee  and  tea  keep  hot  longer  in  brightly  polished  pots,  or 
in  dull,  tarnished  ones?  Why?  What  should  be  the  nature  of  the  - 
surface  of  a stove  in  order  that  the  heat  may  readily  escape  into  the 
room  ? 

What  is  meant  by  selective  absorption  ? By  what  is  it  caused  ? 

What  effect  is  produced  in  the  reflecting,  absorptive,  and  emissive 
powers  of  substances  by  polishing  them?  What  by  roughening  them?  , 
Name  any  diathermanous  substance.  Is  dry  air  diathermanous  or  1 
athermanous?  Is  moist  air  diathermanous  or  athermanous  ? 


CHAPTER  V. 

CHANGE  OF  STATE.  — LATENT  AND  SPE- 
CIFIC HEAT.  — MECHANICAL  EQUIV- 
ALENT OF  HEAT. 

221.  Fusion  and  Solidification. — The  force  of  heat 
is  opposed  to  that  of  cohesive  attraction^  so  that,  if  suf- 
ficient heat  is  added  to  a solid,  it  becomes  changed  into 
a liquid ; while,  on  the  other  hand,  all  liquids,  when 
caused  to  lose  sufficient  heat,  are  changed  into  solids. 
These  processes  are  called /wsfow  and  solidification. 

The  amount  of  heat  in  a body,  as  indicated  by  a 
thermometer,  that  is,  by  the  temperature,  is  gener- 
ally spoken  of  as  the  sensible  heat  of  the  body.  Dur- 
ing a change  of  state,  such,  for  example,  as  the  change 
from  a solid  to  a liquid,  or  from  a liquid  to  a gas  or 
vapor,  considerable  heat  passes  into  the  body  without 
changing  its  temperature.  This  heat,  therefore,  is  dif- 
ferent from  sensible  heat,  and,  since  it  does  not  affect 
i the  temperature,  is  called  latent  heat. 

I 222.  Laws  of  Fusion.- — -When  a solid  has  been 
liquefied  or  melted  by  the  action  of  heat,  we  say  that 
it  has  been  fused.  It  can  be  shown,  experimentally, 

, 1st.  That  under  the  same  pressure  every  substance 
capable  of  fusion  has  a fixed  temperature  at  which  it 
begins  to  fuse. 

; 17 


N 


193 


194 


NATURAL  RHILOSOPHT. 


2d.  That  when  any  substance  begins  to  fuse,  the  tem- 
perature remains  the  same  until  the  whole  of  the  sub- 
stance has  been  fused. 

Thus,  ice  begins  to  melt  or  fuse  at  the  temperature 
of  32°  Fall.  If  a quantity  of  ice  be  placed  in  a vessel 
over  any  source  of  heat,  a thermometer  plunged  in  the 
water  which  comes  from  the  melted  ice,  will  remain  at 
the  same  temperature,  viz.,  at  32°  Fah.,  until  all  of  the 
ice  is  melted. 

223.  L-aws  of  Solidification. — When  bodies  which 
have  been  fused  by  heat  are  sufficiently  cooled,  they 
solidify.  As  solidification  is  merely  the  reverse  of 
liquefaction,  the  laws  of  solidification  are  similar  to 
those  of  fusion,  viz.: 

1st.  Under  the  same  pressure  every  substance  solidifies  | 
at  a certain  temperature,  which  is  the  same  as  that  at 
which  it  fuses.  j 

2d.  When  the  solidification  has  begun,  the  temperature  | 
remains  the  same  until  all  the  substance  has  solidified. 

Thus,  water  begins  to  freeze  at  32°  Fall.,  which  is 
exactly  the  same  temperature  as  that  at  which  ice 
begins  to  melt.  When  a body  of  water  which  has 
been  cooled  to  32°  Fah.  comniences  to  freeze,  no  matter 
how  intense  the  cold,  the  temperature  will  remain  at  ! 
32°  Fah.  until  all  the  water  is  frozen.  | 

224.  Latent  Heat.  — Heat  must  be  passed  into  ice  * 
before  it  can  be  melted,  and  yet  the  water  formed  by  | 
its  melting  has  exactly  the  same  temperature  as  that  i 
of  the  ice  from  which  it  came,  viz.,  32°  Fah.  It  is,  ! 
therefore,  evident  that  all  the  heat  ichich  was  absorbed  < 
during  the  melting  of  the  ice  has  been  rendered  latent. 

In  order  to  cause  water  heated  in  an  open  vessel  to  ! 
212°  Fah.  to  continue  boiling,  it  is  necessary  to  pass  ' 


LATENT  AND  SPECIFIC  HEAT. 


195 


into  it  considerable  beat;  and  yet  tlie  pressure  remain- 
ing the  same,  the  steam  produced  by  the  boiling  of 
the  water  has  exactly  the  same  temperature  as  that  of 
the  water  from  which  it  came,  viz.,  212°  Fah.  It  is, 
therefore,  evident  that  all  the  heat  which  was  absorbed 
during  the  boiling  of  the  water,  that  is,  during  its  change 
from  liquid  to  vapor,  has  been  rendered  latent. 

/^Latent  heat  is  that  which  does  not  increase  the  tem- 
perature, and  cannot,  therefore,  be  detected  by  the  ther- 
mometer.^' 

225.  Latent  Heat  of  Water  at  32°  Fah.  — The 

amount  of  heat  that  is  rendered  latent  during  the 
melting  of  a mass  of  ice  is  sufficient  to  raise  the  tem- 
perature of  one  hundred  and  forty-two  times  as  much 
water  1°  Fah.  This  fact  is  often  expressed  by  saying 
that  142°  of  heat  have  been  rendered  latent. 

This  can  be  shown  as  follows ; A pound  of  ice  and  a pound  of  water, 
each  at  32“  Fah.,  are  placed  in  exactly  similar  vessels,  over  the  same 
source  of  heat,  so  that  in  any  time  each  shall  receive  as  much  heat  as 
the  other.  When  all  the  ice  has  melted,  the  temperature  of  the  water 
formed  therefrom  is  only  32“,  but  the  water  in  the  other  vessel  will  he 
at  174“ ; that  is,  it  has  gained  142“  of  heat.  We  infer,  therefore,  that 
the  ice  required  142“  to  melt  it ; but  since  the  temperature  of  the  water 
so  produced  is  only  32“  Fah.,  then  142“  must  have  been  rendered 
latent. 

226.  Change  of  Latent  into  Sensible  Heat.  — A 

definite  amount  of  heat  is  required  to  raise  the  tem- 
perature of  a body  through  any  number  of  degrees, 
and  when  the  body  so  heated  is  again  cooled  to  its 
original  temperature,  the  same  definite  amount  of  heat 
is  removed.  Whenever,  therefore,  heat  has  become  latent 
in  producing  a change  of  state  in  any  substance,  when 
the  substance  returns  to  its  original  state  the  same  quan- 
tity  of  heat  is  again  evolved. 

Heat  is  rendered  latent  in  changing  a solid  into  a 


196 


NATURAL  PHILOSOPHY. 


liquid,  or  a liquid  into  a vapor  or  gas.  Latent  heat  | 
becomes  sensible,  that  is,  heat  is  evolved,  when  a liquid  ‘ 
solidifies,  or  a vapor  or  gas  is  condensed. 

I 

227.  Effect  of  Freezing  on  the  Temperature  of  j 

the  Air.  — A large  body  of  water  cannot,  in  general,  : 
be  cooled  below  the  temperature  of  32°  Fah.  until  its 
entire  mass  has  been  changed  into  ice.  During  the 
change  from  water  to  ice,  a large  quantity  of  heat  is  | 
given  out  which  was  before  latent  in  the  water,  conse- 
quently, the  cold  air  in  contact  with  the  water,  while 
causing  the  cooling  of  the  water,  itself  becomes  heated.  | 
Therefore,  the  freezing  of  large  bodies  of  water  tends 
to  raise  the  temperature  of  the  air. 

Freezing  and  melting  are  both  gradual  processes,  on  i 
account  of  the  heat  which  disappears  in  the  one  case 
and  reappears  in  the  other. 

228.  Freezing  Mixtures.  — Considerable  heat  is 
rendered  latent  during  the  change  of  a solid  into  a 
liquid  by  solution.  Flence,  when  solids  are  rapidly 
dissolved  in  a liquid,  a marked  coolimj  occurs.  In 
some  cases  the  liquid  is  unarmed ; here,  however,  a 
chemical  combination  has  occurred  between  the  solid 
and  the  liquid,  and  more  heat  has  been  produced  than 
was  absorbed  during  the  solution. 

Advantage  is  taken  of  the  cooling  produced  by  solu- 
tion to  obtain  low  temperatures  artificially.  Freezing 
mixtures  consist  of  various  proportions  of  dift’erent 
solids,  or  of  different  solids  and  liquids,  which,  when 
mixed,  will  dissolve,  and  so  cause  a reduction  of  tem- 
perature. By  their  use  very  low  temperatures  can  be 
obtained. 

One  of  the  simplest  freezing  mixtures  consists  of  one  part  of  salt, 
and  two  parts  of  ice  or  snow,  spread  in  alternate  layers.  ith  this 


LATENT  AND  SPECIFIC  HEAT. 


197 


mixture,  a temperature  as  low  as  — 5°  Fah.,  that  is,  5°  below  the  zero 
of  Fahrenheit’s  scale,  can  be  obtained.  Freezing  mixtures  are  used  in 
the  preparation  of  ice-cream.  The  material  to  be  frozen  is  put  in  a 
tin  vessel  placed  inside  a larger  vessel  of  wood.  Salt  and  ice  are 
packed  in  layers  between  the  two  vessels.  The  salt  causing  the  ice 
to  melt  rapidly,  the  heat  necessary  for  the  melting  of  the  ice  is  taken 
from  the  cream,  which  is  thus  frozen.  The  outer  vessel  is  made  of 
some  bad  conductor  of  heat,  such  as  wood,  so  as  to  prevent  the  ice 
obtaining  any  heat  from  the  air. 

229.  Increase  of  Volume  during  Solidification. — 

Many  metals,  such  as  mercury,  contract  on  solidifying; 
the  freezing  of  the  mercury  in  the  bulb  of  a ther- 
mometer does  not,  therefore,  burst  the  bulb.  Other 
metals,  such  as  antimony,  are  believed  to  expand  on 
solidifying,  and  hence  fill  the  moulds  into  which  they 
have  been  poured,  and  so  take  sharp  casts.  Water 
also  expands  on  freezing,  and  does  so  with  considerable 
force.  Wash-tubs  filled  with  water  are  often  burst 
daring  a cold  night ; even  iron  bomb-shells  have  been 
burst  by  filling  them  with  water,  and  exposing  them 
to  a freezing  temperature. 

' When  water  is  absorbed  by  porous  rocks,  or  runs  into  the  crevices  of 
those  of  more  compact  structure,  on  freezing,  it  expands  with  sufficient 
force  to  break  the  rocks  into  fragments.  It  is  in  this  way  that  water 
I acts  to  degrade  or  break  down  the  rocks,  and  so  aids  the  rivers  in 
carrying  the  mountains  towards  the  ocean. 

‘ 230.  Vaporization. — When  most  liquids  are  suffi- 

ciently heated,  they  become  changed  into  vapors. 
; When  the  vapor  is  given  off  from  only  the  surface, 
: the  liquid  is  said  to  evaporate ; but  when  the  vapor  is 
: given  off  from  below  the  surface  also,  the  liquid  is 
said  to  hoil.  Some  solids  pass  into  a state  of  vapor 
! without  appearing  to  first  become  liquids ; they  are 
then  said  to  be  sublimed.  Arsenic  is  an  example  of 
such  a solid. 


198 


NATURAL  PHILOSOPHY. 


231.  Formation  of  Vapors  in  a Vacuum. — If  any 

volatile  liquid  be  put  into  a vacuous  space,  it  'vvill 
rapidly  pass  into  the  state  of  a vapor  without  the 
addition  of  external  heat. 

If  a drop  of  any  volatile  liquid  be  passed  up  into 
the  empty  space  above  the  mercury  in  a barometer 
tube,  we  will  notice  not  only  that  the  liquid  will  dis- 
appear and  turn  into  vapor,  but  that  at  the  same 
time  the  vapor  zvill  more  than  fill  the  vacuum.,  that  is, 
that  it  will  depress  the  mercury  column,  thus  showing 
that  it  possesses  tension.  If,  now,  a little  more  liquid 
be  passed  into  the  tube,  it  will  also  evaporate,  and  the 
column  of  mercury  will  be  further  depressed ; but  after 
a certain  amount  of  vapor  has  been  formed,  the  amount 
of  which  depends  on  the  temperature,  the  mercury 
column  will  remain  stationary,  and  no  more  liquid  will 
be  evaporated ; and  if  it  be  passed  into  the  tube,  it  will 
simply  float  on  top  the  mercury.  The  vapor  in  the 
tube  is  then  at  its  maximum  or  greatest  tension,  and 
the  space  it  occupies  is  saturated  icith  vapor.  If,  now, 
the  tube  be  heated,  more  of  the  liquid  will  evaporate, 
and  the  mercury  will  be  further  depressed. 

232.  Circumstances  Influencing  Evaporation.  — 
The  rapidity  with  which  a liquid  evaporates  into  the 
air  depends 

1st.  On  the  extent  of  surface,  becanse  evaporation 
takes  place  at  the  surface. 

2d.  On  the  quantity  of  the  same  vapor  already  pres- 
ent in  the  air,  because,'  when  the  air  is  saturated,  no 
more  of  the  liquid  can  evaporate. 

3d.  On  the  removal  of  the  air.  Evaporation  ceases 
when  the  air  over  the  liquid  is  saturated;  if,  however, 
fresh  air  is  brought  to  the  surface,  more  liquid  evap- 
orates. 


LATENT  AND  SPECIFIC  HEAT. 


199 


4111.  On  the  temperature^  because  warm  air  can  hold 
more  vapor  than  cold  air. 

5th.  On  tlie  pressure  on  the  surf  ace.  The  greater  the 
pressure  the  greater  the  evaporation.  In  a vacuum 
all  vaporizable  liquids  rapidly  vaporize. 

233.  Laws  of  the  Boiling  of  Liquids.  — The  laws 
for  the  boiling  of  liquids  are  similar  to  those  for  the 

i fusion  of  solids.  They  may  be  expressed  as  follows : 

1st.  The  pressure  remaining  the  same,  there  is  for  every 
I liquid  a certain  temperature  at  which  it  hoils. 

2d.  When  a liquid  has  been  heated  to  the  boiling-point, 
the  temperature  remains  the  same  until  all  the  liquid  has 
been  vaporized.  All  the  heat  a liquid  receives,  after  it 
has  once  reached  its  boiling-point,  is  rendered  latent 
in  converting  it  into  vapor. 

234.  Circumstances  Influencing  the  Boiling-Point. 

— An  increase  of  pressure  raises  the  boiling-point.  Before 
a liquid  exposed  to  the  air  can  boil,  its  vapor  must 
i have  a tension  sufficient  to  enable  it  to  overcome  the 
pressure  of  the  air.  Hence,  the  tension  of  the  vapor  es- 
caping from  a boiling  liquid  is  equal  to  the  pressure  of 
the  atmosphere.  If,  therefore,  the  pressure  on  a liquid 
be  increased,  the  tension  of  its  vapor  must  be  increased 
I before  the  liquid  can  boil ; that  is,  the  temperature 
! of  the  liquid  must  be  increased. 

j A liquid  can  never  be  raised  above  the  temperature 
of  its  boiling-point,  if  its  vapor  is  allowed  to  escape 
! into  the  air.  If  the  vapor  be  confined,  the  pressure 
1 on  the  surface  is  increased,  and  the  temperature  can 
I be  increased  to  any  extent,  provided  the  vessel  is  suffi- 
I ciently  strong.  To  extract  glue  from  bones,  they  are 
I boiled  at  very  high  temperatures,  in  water  placed  in 
closed  vessels  provided  with  safety-valves  for  the  es- 


200 


NATURAL  PHILOSOPHY. 


cape  of  the  steam,  should  the  pressure  become  too 
great. 


Experiment.  — That  a diminished  pressure  lowers  the  boiling  point 
may  be  shown  by  an  experiment  that  is  sometimes 
called  the  culinary  paradox.  Water  is  boiled  in  a 
suitable  glass  flask,  A,  and  after  a few  minutes  of 
vigorous  boiling,  so  as  to  permit  the  steam  formed 
to  drive  all  the  air  out  of  the  flask,  the 
source  of  heat  is  removed,  and  the  neck 
is  closed  by  a tightly-fitting  cork,  which 
has  been  previously  steeped  in  melted  wax 
or  paraffin,  so  as  to  fill  all  its  pores.  The 
vessel  is  now  inverted  below  a water-sur- 
face, C,  to  prevent  the  entrance  of  air.  For 
a few  moments  the  water  will  continue  to 
boil ; but  the  increased  pressure  on  the  sur- 
face produced  hy  the  confined  vapor  soon 
raises  the  hoiling-poini  and  stops  the  hailing. 

Fig.  9^  Igj.  goi-fie  cold  water  fall  on  the  bottom 

The  Colinary  Paradox.  ...in.  ^ -n-  no  t-u 

of  the  flask,  as  shown  m Fig.  93.  The  va- 
por will  then  be  condensed,  and  the  pressure  being  diminished,  the 
liquid  bursts  into  vigorous  boiling.  Pour  hot  water  on  the  flask  and 
it  will  again  stop  boiling. 

The  vapor  which  escapes  into  the  air  from  boiling  liquid,  has  at  the 
ordinary  pressure  of  the  air  always  the  same  temperature  — viz.,  212° 
Fah. 


Fig.  94. 

Water  Boiled  in  a 
Paper-Bag. 


That  the  temperature  of  the  liquid 
does  not  rise  above  the  boiling-point 
may  be  shown  by  the  following  equally 
curious  operation. 

Experiment. — To  boil  water  in  a paper-hag.  Take 
a square  piece  of  paper  and  fold  it  so  as  to  form  a 
conical  bag.  A,  as  shown  in  Fig.  94.  Suspend  the 
bag  by  strings,  and,  pouring  water  into  it,  allow  the 
flame  of  an  alcohol-lamp  or  Bunsen  burner  to  fall 
on  the  bag,  being  careful  to  prevent  the  flame  from 
touching  the  paper  in  any  place  where  there  is  no 
water.  The  water  can  now  be  heated  until  it  boils, 
without  the  paper  being  burned,  because  the  paper 


LATENT  AND  SPECIFIC  HEAT. 


201 


cannot  be  heated  much  more  than  212°  Fah.,  and  this  is  not  sufBcient 
to  burn  it. 

Caution. — Select  good  writing-paper,  moderately  free  from  sizing. 
Do  not  crease  the  paper  in  folding.  Avoid  letting  the  flame  play  too 
much  on  the  point  of  the  cone. 

InfMence  of  adhesion  on  the  boiling-point.  — The  press- 
ure remaiuing  the  same,  the  boiling-point  varies  slightly 
with  the  nature  and  shape  of  the  vessel  in  wdiich  the 
liquid  is  boiled ; because,  before  the  vapor  can  escape, 
the  adhesion  of  the  liquid  to  the  vessel  must  be  over- 
come. Solids  dissolved  in  a liquid  increase  the  tem- 
perature of  the  boiling-point  for  the  same  reason ; be- 
cause the  vapor  which  escapes  does  not  carry  the  solid 
with  it. 

Distillation  is  a process  by  which  a liquid  can  be 
separated  from  a solid  dissolved  in  it.  We  boil  the 
solution  and  condense  the  vapor  as  it  escapes. 

235.  Latent  Heat  of  Vapors. — The  amount  of  heat 
rendered  latent  during  the  change  of  a given  quantity 
of  water  at  212°  into  steam  at  212°,  is  sufficient  to 
raise  the  temperature  of  about  one  thousand  times  as 
much  water  1°  Fah.  This  is  often  expressed  by  saying 
that  the  water  has  absorbed  1000°  of  heat,  or  that 
1000°  degrees  have  been  rendered  latent. 

When  vapor  loses  heat  and  condenses,  the  latent  heat 
again  appears  as  sensible  heat. 

236.  Reduction  of  Temperature  Caused  by  Evap- 
oration.— We  are  cooled  by  fanning,  because  the  warm 
air  thus  brought  into  contact  with  the  skin  causes  a 
rapid  evaporation  of  the  moisture  on  the  skin,  and 
thus  lowers  the  temperature. 

If  water  be  placed  in  a vacuous  space,  and  the  vapor 
which  escapes  from  it  removed  as  rapidly  as  it  is 
formed,  the  water  will  at  last  be  frozen  by  its  own 


202 


NATURAL  PHILOSOPHY. 


evaporation.  Ice  macliines  are  constructed  on  this 
principle.  The  water  must  have  heat  in  order  to 
evaporate,  and  this  heat  is  taken  from  the  rest  of  the 
water,  which  is  thus  frozen. 

During  the  change  of  water  into  vapor  bj  the  heat 
of  the  sun,  more  than  1000°  of  heat  are  rendered 
latent.  This  heat  again  becomes  sensible  as  the  water 
condenses.  Since  a large  amount  of  this  condensation 
takes  place  in  cool  or  cold  countries,  we  can  see  that 
such  countries  must  be  made  warmer  bj  means  of  the 
rain  or  snow  which  falls  in  them. 

237.  Specific  Heat. — Equal  quantities  of  different 
substances  are  not  raised  through  the  same  number  of 
degrees  of  temperature,  by  the  addition  of  the  same 
quantity  of  heat.  Thus,  if  we  add  the  same  amount 
of  heat  to  a pound  of  mercury  and  to  a pound  of  water, 
we  will  find  that  if  the  water  has  been  heated  one  de- 
gree, the  mercury  will  have  been  heated  thirtv-three 
degrees.  We  therefore  conclude  that  water  has  thirty- 
three  times  the  capacity  for  heat  that  mercury  has,  or 
that  thirty-three  times  more  heat  is  necessary  to  in- 
crease the  temperature  of  a given  weight  of  Avater  one 
degree  than  is  required  to  increase  the  temperature  of 
an  equal  weight  of  mercury  one  degree. 

By  the  specific  heat  of  a substance  Ave  mean  the 
amount  of  heat  required  to  raise  the  temperature  of  a 
given  quantity  of  that  substance  through  a certain 
number  of  degrees,  as  compared  AA'ith  the  amount  of 
heat  required  to  raise  an  equal  quantitv  of  some  other 
substance  through  the  same  number  of  degrees.  W ater 
is  the  substance  generally  adopted  as  the  unit  of  com- 
parison. Specific  heat,  like  specific  gravity,  is  merely 
a ratio. 


LATENT  AND  SPECIFIC  HEAT. 


203 


• 238.  The  Calorimeter.  — The  thermometer  only 

! indicates  the  change  -which  has  taken  place  in  the 
temperature  of  a body,  that  is,  it  only  gives  the  sen- 
! sible  heat.  To  obtain  the  specific  heat,  -we  must  kno-w 
f the  latent  as  -well  as  the  sensible  heat.  There  are 
^ various  methods  of  doing  this,  one  of  the  best  of 
( which  is  by  means  of  the  caloriraeter. 


I 

I 


I 

I 


i 


We  can  ascertain  the  total  amount  of  heat  given  out  by  a body  in 
cooling,  from  any  given  temperature  to  that  of 
melting  ice,  by  seeing  how  much  ice  it  will  melt. 

The  calorimeter  consists  of  three  hollow  vessels, 

M,  A,  and  £,  placed  inside  one  another,  as  shown 
in  Fig.  95.  The  vessels  A and  B are  packed  with 
dry  ice.  The  substance,  whose  specific  heat  is  de- 
sired, is  placed  in  M,  and  in  cooling  melts  the  ice 
in  A,  the  quantity  melted  being  inferred  from  the 
water  which  runs  into  D.  The  ice  in  B prevents 
the  heat  of  the  outer  air  from  melting  any  of  the 
ice  in  A.  Suppose  one  pound  of  a given  substance 
is  heated  to  212°  and  placed  in  M,  and  in  cooling 
to  32°  only  melts  half  as  much  ice  as  a pound  of 
water  would  in  cooling  from  212°  to  32°  — then 
the  specific  heat  of  the  body  is  | = .5. 


Fig.  95. 

The  Calorimeter. 


j 239.  The  Specific  Heat  of  Water. —Water  has  a 
[ higher  specific  heat  than  almost  any  other  common  sub- 
j stance,  that  is,  it  takes  in  more  heat  in  being  warmed, 
i and  gives  out  more  in  cooling,  than  any  common  sub- 
stance. Since  about  three-fourths  of  the  earth’s  sur- 
j face  is  covered  with  water,  we  can  see  that  the  high 
i specific  heat  of  water  must  exert  a great  influence  in 
I preventing  extremes  of  temperature,  since  water  can 
absorb  or  emit  considerable  heat  without  much  change 
I in  temperature. 


j 240.  Mechanical  Equivalent  of  Heat.  ^ ^ Energy 
can  never  be  destroyed.  When  it  disappears  in  one 
j form  it  reappears  as  another.  Heat  is  one  of  the  com- 


204 


NATURAL  PHILO  SOPHY. 


monest  forms  into  which  mechanical  motion  can  he  i 
changed;  for,  since  lieat  is  an  effect  produced  by  tbe  i 
vibrations  or  shakings  of  the  molecules  of  bodies,  we 
can  easily  change  mechanical  motion  into  heat.  This 
fact  was  first  discovered  by  an  American  named  Ben- 
jamin Thomson,  but  afterwards  called  Count  Eumford. 

We  know,  as  the  result  of  many  accurate  experi- 
ments, the  exact  amount  of  mechanical  force  neces-  I 
sary  to  produce  a given  quantity  of  heat.  j 

The  force  necessary  to  •produce  sufficient  heat  to  raise  j 
the  temperature  of  one  pound  of  water  1°  Fah.,  is  equal 
to  that  produced  by  a weight  of  T12  lbs.  falling  through  j 
the  space  of  one  foot.  Or,  conversely,  one  pound  of  j 
water  in  cooling  through  1°  Fah.  gives  out  a quantity  i 
of  heat  which  is  capable  of  exerting  a mechanical  force  I 
sufficient  to  raise  772  lbs.  through  the  space  of  one  , 
foot.  These  figures  were  first  determined  by  an  Eng-  ■ 
lishman  named  Joule,  and  are  called  Joule's  equivalent.  \ 

The  fact  that  heat  can  be  produced  by  mechanical  ! 
force  is  seen  in  the  heat  developed  by  tbe  friction  of  ' 
one  surface  on  another,  and  also  in  the  heat  developed  i 
by  percussion.  A stout  copper  wire,  if  rubbed  with  a i 
piece  of  stiff  paper,  may  be  made  hot  enough  to  set  fire  ! 
to  a match;  or  a soft  iron  nail  may  be  made  sufficiently 
hot,  by  rapid  hammering,  to  char  a piece  of  paper. 

241.  The  Steam-Engine. — The  method  most  com- 
monly adopted  for  the  change  of  heat  into  mechanical 
work,  is  by  means  of  the  steam-engine. 

The  heat  is  employed  to  change  water  into  steam. 
The  water  is  placed  in  a suitable  vessel,  generally 
made  of  iron,  called  the  boiler.^  the  construction  of 
which  varies  with  the  character  of  the  steam-engine 
with  which  it  is  to  be  used. 


LATENT  AND  SPECIFIC  HEAT. 


205 


The  steam  passes  from  tlie  boiler  through  a pipe 
leading  to  a box,  Fig.  96,  called  the  steam-chest, 
through  which,  by  a con- 
trivance, B,  called  the 
slide-valve,  it  is  admitted 
alternately  to  different 
sides  of  a piston,  C,  so  ar- 
ranged as  to  move  freely 
in  the  cylinder,  D. 

By  the  pressure  which  x 
the  steam  exerts  on  the 
piston,  it  is  moved  back- 
wards and  forwards  from 
one  end  of  the  cylinder 
to  the  other.  The  mo- 
tion of  the  piston  is  com- 
municated by  means  of 


Fig.  96. 

The  Low-Pressnie  Steam-Engine. 


the  rod,  E,  to  a beam,  F,  moving  on  a hearing,  G.  By 
means  of  the  connecting-rod,  H,  attached  to  the  other 
end  of  the  beam,  R,  the  motion  is  carried  to  a large 
heavy  wheel,  I,  called  the  fly-wheel.  The  alternating 
motion  of  the  beam  is  converted  into  a steady  rotary 
motion  by  means  of  the  crank,  J.  In  this  manner  the 
backward  and  forward  motion  of  the  piston  is  caused 
to  produce  a continued  rotary  motion  of  the  fly-wheel. 
To  the  axis  of  the  fly-wheel  pulleys  are  generally  at- 
tached, from  which,  by  means  of  belting  and  shafting, 
the  motion  is  carried  to  the  machinery  to  be  moved. 

By  means  of  the  slide-valve  the  steam  in  the  steam- 
chest  is  alternately  cut  off  from  one  side  of  the  piston 
and  admitted  to  the  other  side,  and  at  the  same  time 
an  opening  provided  through  which  the  steam  can 
escape  from  that  side  of  the  piston  from  which  the 
steam  has  been  cut  off.  In  the  low-pressure  steam- 
18 


206 


NATURAL  rniLOSOPHT. 


engine  this  steam  passes  through  a pipe,  into  a 
chamber,  called  the  condenser.  In  this  chamber  a 
jet  of  cold  water  is  allowed  to  play.  By  this  means  j 
the  steam  which  passes  from  the  steam-cylinder  into  i 
the  condenser  is  condensed,  thereby  lowering  the  ! 
pressure  on  that  side  of  the  piston  from  which  the  : 
steam  has  been  cut  off.  A pump,  J/,  called  the  air- 
pump,  worked  by  the  rod,  0,  removes  the  water  from 
the  condenser.  The  slide-valve  is  moved  by  means 
of  a bent  lever  moved  by  the  eccentric  rod,  P. 

In  the  high-pressure  engine,  the  steam  escapes  at 
once  into  the  air,  after  it  has  moved  the  piston  to 
either  end  of  the  cylinder.  The  puffs  of  steam  which 
escape  from  such  engines  denote  the  speed  with 
which  the  piston  is  being  driven  backwards  and  for- 
wards. 

When  the  piston  is  at  the  farthest  part  of  its  stroke, 
that  is,  when  it  is  at  one  end  or  the  other  of  the  cyl- 
inder, the  crank,  J,  and  connecting-rod,  H,  are  in  the 
same  straight  line.  In  such  a position  the  tendency 
to  motion  of  the  beam,  F,  will  not  be  carried  through 
the  connecting-rod  and  crank  so  as  to  produce  a rota- 
tion of  the  fly-wheel,  but  will  simply  produce  a strain 
on  those  parts.  These  two  positions  are  called  the 
dead  points  of  the  engine.  The  fly-wheel,  by  its 
inertia,  continues  to  move  and  carries  the  crank  past 
these  points. 

242.  Other  Sources  of  Heat. — Besides  the  sources 
of  heat  already  mentioned,  we  have  heo.t  of  the  sun  and 
fixed  stars,  and  that  produced  by  chemical  combination 
and  by  electricity. 

The  way  in  which  heat  is  produced  by  chemical  com- 
bination is  well  illustrated  by  the  case  of  a body  burn- 


SYLLABUS. 


207 


ing  in  the  air.  As  the  combustible  body  combines 
with  the  oxygen  of  the  air,  the  oxygen,  in  rushing 
towards  the  combustible  materials  in  the  body,  sets 
its  molecules  into  the  vibratory  motion  necessary  to 
cause  heat. 

Syllabus. 

When  a solid  is  changed  into  a liquid  by  the  addition  of  heat,  it  is 
said  to  he  fused.  When  the  fused  liquid  is  allowed  to  cool,  it  again 
becomes  solid. 

Sensible  heat  is  the  heat  a body  possesses  as  indicated  by  its  tem- 
perature, or  by  the  thermometer.  Heat  which  does  not  raise  the  tem- 
perature is  called  latent  heat. 

In  order  to  cause  a mass  of  ice  at  32°  Fah.  to  melt,  a quantity  of  heat 
is  rendered  latent,  which,  were  it  acting  as  sensible  heat,  would  be  suf- 
ficient to  raise  the  temperature  of  142  times  as  much  water  1°  Fah. 

Heat  is  rendered  latent  in  changing  a solid  into  a liquid  or  a liquid 
into  a gas.  Latent  heat  becomes  sensible,  or  heat  is  evolved,  when  a 
liquid  solidifies  or  a gas  is  condensed, 
j Freezing  mixtures  consist  of  mixtures  of  various  solids,  or  solids  and 
j liquids  which,  when  brought  together,  melt  or  dissolve  rapidly.  The 
! reduction  of  temperature  they  cause  is  due  to  the  heat  which  is  ren- 
; dered  latent. 

^ Some  substances,  like  water  and  antimony,  at  the  moment  of  solidi- 
ij  fying  expand  with  considerable  force. 

[ A liquid  is  said  to  evaporate  when  it  gives  off  vapor  from  the  sur- 
I face  only  ; it  is  said  to  boil  when  it  gives  off  vapor  both  at  and  from 
I below  the  surface. 

All  vaporizable  liquids  vaporize  at  once  when  placed  in  a vacuum, 
j:  When  a given  space  holds  as  much  vapor  as  it  can  be  made  to  hold 
f without  change  of  temperature,  it  is  said  to  be  saturated. 

[ The  rapidity  of  evaporation  is  influenced,  1st.  By  the  extent  of  sur- 
face ; 2d.  By  the  amount  of  the  same  vapor  already  in  the  air ; 3d. 
E By  the  renewal  of  the  air  ; 4th.  By  the  temperature,  and,  5th.  By  the 
1 pressure  on  the  surface. 

When  a liquid  begins  to  boil,  the  temperature  remains  the  same 
I until  the  whole  has  been  vaporized.  The  vapor  which  escapes  from 


of  the  liquid. 


208 


NATURAL  PUILOSOPEY. 


The  boiling-poiut  is  increased  by  an  increase  of  pressure.  It  i.o  also 
increased  by  adhe.sion,  whether  between  the  liquid  and  the  walls  of 
the  vessel  or  between  the  liquid  and  solids  in  solution. 

The  reduction  of  temperature  produced  by  evaporation  is  caused  by 
the  large  amount  of  heat  that  becomes  latent  when  a liquid  is  changed 
into  a vapor. 

By  the  specific  heat  of  a substance,  we  mean  the  amount  of  heat  re- 
quired to  raise  the  temperature  of  a given  quantity  of  the  substance 
through  a given  number  of  degrees,  as  compared  with  the  amount 
of  heat  required  to  raise  the  weight  of  an  equal  quantity  of  water 
through  the  same  number  of  degrees. 

The  sensible  heat  of  a body  is  determined  by  a thermometer ; the 
specific  heat  by  a calorimeter. 

Water  possesses  the  greatest  specific  heat  of  any  common  substance. 

By  the  mechanical  equivalent  of  heat  we  mean  the  amount  of  me- 
chanical force  which  a given  quantity  of  heat  is  capable  of  exerting. 

772  lbs.  falling  through  the  space  of  one  foot,  represent  a force  ca- 
pable of  heating  1 lb.  of  water  1°  Fah. 

Heat  may  be  caused  to  give  up  its  energy  in  the  form  of  mechanical 
work  by  means  of  the  steam-engine. 

The  principal  sources  of  heat  are,  1st.  Mechanical ; 2d.  Heat  of  sun 
and  fixed  stars : 3d.  Chemical  combination ; 4th.  Electricity. 

Questions  for  Review. 

What  is  the  difference  between  latent  and  sensible  heat? 

State  the  laws  for  the  fusion  and  for  the  solidification  of  solids. 

How  many  degrees  of  heat  are  rendered  latent  during  the  melting 
of  ice  ? Explain  fully  how  this  can  be  determined. 

Under  what  circumstances  is  sensible  heat  rendered  latent  ? Under  : 
what  circumstances  is  latent  heat  rendered  sensible  ? 

WTiat  are  freezing  mixtures?  Explain  the  manner  in  which  they  | 
act.  Name  any  simple  freezing  mixture  in  common  use.  i , 

Name  any  substances  that  increase  in  volume  on  solidifying.  Name  | i 
any  substance  which  decreases  in  volume  on  solidifying.  How  does  | ' 
water  cause  the  degradation  or  breaking  down  of  rocks?  j 

When  is  a liquid  said  to  evaporate?  When  is  it  said  to  boil?  |i 
When  is  a solid  said  to  be  sublimed?  j 

What  happens  when  a volatile  liquid  is  put  into  a vacuum?  De-  1 1 
scribe  an  experiment  which  shows,  1st.  That  vapors  possess  tension,  ; i 


QUESTIONS  FOR  REVIEW. 


209 


and  2d.  That  when  a space  is  saturated  with  any  vapor,  no  more  of 
the  liquid  will  evaporate. 

Name  the  circumstances  which  influence  the  rapidity  of  evapora- 
tion. Explain  why  each  acts  in  the  manner  it  does. 

State  the  laws  for  the  boiling  of  liquids.  How  much  heat  is  ren- 
dered latent  during  the  change  of  water  at  212°  to  steam  at  212°  ? 
Name  the  circumstances  which  affect  the  boiling  point.  Describe  the 
culinary  paradox. 

How  can  water  be  boiled  in  a paper-bag?  Why  does  not  the 
paper  burn  ? 

Name  some  examples  of  the  reduction  of  temperature  caused  by 
evaporation. 

Define  specific  heat.  How  may  the  specific  heat  of  a substance  be 
determined  ? Describe  the  construction  of  the  calorimeter. 

What  effect  is  produced  on  the  climate  of  the  earth  by  the  very 
high  specific  heat  of  water  ? Why  should  this  effect  be  produced  ? 

What  do  you  understand  by  the  mechanical  equivalent  of  heat? 
What  is  the  value  of  Joule’s  equivalent?  Who  discovered  the  rela- 
tion that  exists  between  heat  and  mechanical  force  ? 

Give  any  examples  in  which  heat  is  produced  by  the  exertion  of 
mechanical  force. 

Name  the  principal  sources  of  heat. 

18*  O 


Part  IV. 

Light  and  Electricity. 


CHAPTER  I. 

LIGHT.  — ITS  NATURE  AND  SOURCES.— AC- 
TION OF  MATTER  ON  LIGHT. 

243.  The  Nature  of  Light.  — Light  is  caused  by  a 
vibratory  motion  of  tbe  luminiferous  ether.  1 

As  we  have  already  seen,  hot  bodies  radiate  or  give  j 
off  their  heat  by  means  of  waves  imparted  by  their  j 
molecules  to  the  surrounding  ether.  When  this  ether- 
motion  is  sufficiently  rapid,  it  becomes  visible  as  light 
The  invisible  ether-motion  constitutes,  in  most  cases, 
radiant  heat,  although  the  visible  rays  also  possess 
heating  power.  Neither  the  visible  mr  the  invisible  ether- 
motion,  however,  is  to  be  confounded  with  temperature, 
which  in  all  cases  is  caused  by  the  vibration  of  the  mole-  | 
cules  of  the  body,  and  never  by  the  vibrations  of  the  ether.  1 

It  is  only  the  atmospheric  vibrations  between  certain  rates  that  are  j 
able  to  excite  the  sensation  of  sound.  These  limits,  as  we  have  seen,  I 
extend  from  16  to  about  48,000  per  second.  Atmospheric  vibrations  i 
less  rapid  than  16,  or  more  rapid  than  about  48,000,  fail  to  affect  the  j 
ear  as  sound,  although  they  differ  from  those  which  can  so  affect  it  in  ; 
no  other  respect  than  as  to  their  wmve  length. 

The  same  is  true  of  the  ether-waves ; only  those  between  certain 

210 


LIGHT— ITS  NATURE  AND  SOURCES.  211 


rates  are  able  to  cause  the  sensation  of  light.  When  either  too  slow 
or  too  rapid,  they  fail  to  affect  the  eye,  and  are  then  invisible. 

244.  Sources  of  Light. — The  principal  sources  of 
light  are  the  sun  and  fixed  stars,  chemical  combina- 
tion, and  electricity.  Nearly  all  our  light  comes  from 
the  sun.  Artificial  light  is  generally  obtained  by  some 
form  of  combustion,  such  as  that  produced  by  burning 
gas  or  oil  in  air. 

245.  Luminous  and  Illuminated  Bodies. — A body 
which  produces  the  light  it  gives  off  is  called  a lumi- 
nous body.  A lighted  candle  is  a luminous  bodjr,  since 
it  produces  or  causes  the  light  it  gives  off  A body 
which  shines  by  throwing  off  light  it  has  received  from 
a luminous  body,  is  said  to  be  illuminated.  The  sun  is 
a luminous,  the  moon  an  illuminated  body.  Nearly  all 
visible  objects  are  illuminated  ; w'e  see  them  by  means 
of  the  light  they  receive  from  luminous  bodies. 

I 246.  Transparent,  Translucent,  and  Opaque  Bod- 
: ies.  — When  a body  allows  light  to  pass  through  it  so 
! as  to  enable  us  to  see  clearly  the  outlines  of  other  bod- 

Iies  through  it,  it  is  said  to  be  transparent ; when  it 
allows  the  light  to  pass  through  it  so  as  not  to  permit 
, us  to  see  the  outlines  of  other  bodies  through  it,  it  is 

I said  to  be  translucent ; when  it  will  not  allow  the  light 
to  pass  through  it  at  all,  it  is  said  to  be  opaque.  The 
difference  between  a transparent  and  a translucent  body 
is  not  so  much  in  the  amount  of  light  which  can  pass 
through  each,  as  in  the  manner  in  which  the  light  q^asses 
i through.  W ater  is  transparent ; oiled  paper  is  trans- 

! lucent ; and  iron  and  wood  are  opaque. 

! 

I Many  substances  that  are  ordinarily  opaque  are  partially  trans- 
i parent  when  in  very  thin  films ; thus,  a thin  film  of  gold  allows  yel- 
11  lowish-green  light  to  pass  through  it,  and  a thin  film  of  silver  a bluish 
I light. 


212 


NATURAL  PHILO  SOPHY. 


24:7.  Ray,  Beam,  and  Pencil.  — A ray  is  a single  i 
line  of  light,  taken  in  the  direction  in  which  the  light  | 
is  moving  ; a beam  is  a number  of  parallel  rajs  ; a j 
pencil  is  a number  of  rays  from  any  luminous  point;  | 
the  pencil  is  converging  when  the  rays  are  all  moving  I 
towards  the  same  point ; and  diverging  when  they  are  all  i 
moving yVom  the  same  point. 

The  particles  of  air  in  a sound-wave  vibrate  in  the  same  direction  in 
which  the  wave  is  moving;  but  in  light  and  heat  the  ether  particles 
vibrate  at  right  angles  to  the  direction  in  which  the  wave  is  moving. 

24:8.  Direction  in  which  Light  Moves.  — Like 
sound  and  heat,  light  moves  in  perfectly  straight  lirm, 
if  the  medium  through  which  it  is  passing  continues  ' 
of  the  same  kind  and  density  throughout. 

A beam  of  light  coming  into  a darkened  room  lights 
up  the  dust  particles  floating  in  the  air,  and  we  can 
then  see  that  the  light  moves  in  straight  lines. 

24:9.  Shadows.  — When  light  falls  on  an  opaque 
body,  the  space  immediately  behind  the  body  into 
which  the  light  cannot  penetrate  is  called  a shadow. 

Shadows  result 
from  the  fact 
that  light  moves 
in  straight  lines, 
and  is  not  per-  ■ 
ceptibly  bent  on 
Fig.  97.  — Umbra  or  Complete  Shadow.  passing  the  edge 

of  a body.  If  a luminous  point,  s,  Fig.  97,  be  placed 
near  an  opaque  body,  A,  the  light  falling  on  the 
opaque  body  will  illumine  the  parts  nearest  it,  but 
grazing  the  edges  of  the  body,  as  at  a and  h.  will 
continue  moving  in  sensibly  the  same  straight  lines, 
s a and  s h,  in  which  it  came  from  the  luminous  point. 


I LIGHT— ITS  NATURE  AND  SOURCES.  213 

' If  the  luminous  point  come  nearer  the  opaque  body,  as 
at  s',  the  shadow  becomes  larger,  being  now  bounded 
^ by  the  lines  s'  a a'  and  s'  h h' . The  shape  of  the  shadow, 
therefore,  is  dependent  on  the  shape  of  the  opaqxie  body,  and 
I the  size  of  the  shadow  on  tlte  distance  of  the  luminous  point 
{ from  the  opaque  body. 

I*  When  the  luminous  body  has  any  extent,  as  for 
1 example,  S,  Fig.  98,  the  light  from  its  central  part 
! meets  the  opaque 
j body,  which  casts 
, a shadow  of  the 
■ same  shape  as  be- 
j fore.  Onlya  part 
! of  this  shadow,  however,  is  complete,  viz.,  that  lying 
; within  the  conical  space  a o b.  The  shadow  is  less  and 
less  complete  as  we  pass  at  P P outside  this  space,  since 
these  portions  are  illumined  by  the  light  coming  from 
the  edges,  c d,  of  the  luminous  body.  That  part  of  the 
shadow  lying  within  a o b,  into  which  no  light  pene- 
I trates,  is  called  the  nmbra  or  complete  shadow ; that  lying 
j without  these  lines,  as  far  as  and  b g,  is  called  the 
I penumbra  or  partial  shadow. 

! Experiment. — Hang  a wet  sheet  from  the  ceiling,  to  act  as  a curtain 

i or  screen.  Where  convenient,  it  is  preferably  hung  in  an  open  door- 
i way  leading  into  an  adjoining  room.  Place  a lighted  candle  on  the  floor 
back  of  the  sheet,  and  then  walk  backwards  and  forwards  between 

I the  candle  and  the  sheet,  and  very  curious  and  grotesque  shadows 
appear  to  those  on  the  other  side  of  the  sheet,  without  their  being 
able  to  see  how  they  were  caused.  While  walking  towards  the  candle 
I the  shadow  rapidly  increases  in  size,  and  while  walking  away  from  it, 

; rapidly  decreases.  By  stepping  over  the  candle,  the  shadow  appears 
I to  be  leaping  through  the  ceiling. 

i 250.  Velocity  of  Light.  — Light  moves  with  the 
almost  inconceivable  velocity  of  about  185,000  miles  a 
second.  This  velocity  could  take  it  more  than  seven 


214 


NATURAL  PHILOSOPHY. 


times  around  the  earth  at  the  equator,  in  a second.  For 
all  distances  on  the  earth  at  which  objects  are  visible,  we 
may  regard  the  transmission  of  light  as  instantaneous. 

The  velocity  of  light  has  been  determined  by  various  astronomical 
observations.  It  has  also  been  measured  by  different  instruments  es- 
pecially contrived  for  the  purpose. 

251.  Actions  which  Take  Place  at  the  Surfaces 
of  Bodies.  — When  light  falls  on  a body,  it  either 
passes  into  the  body,  or  is  thrown  otf  from  its  surface. 

The  light  Avhich  is  thrown  oft'  from  the  surface  is 
either  diffused  or  reflected. 

The  light  which  passes  into  the  body  may  pass 
through  it,  if  the  body  be  transparent  or  translucent; 
if  in  this  case  the  direction  of  the  light  is  changed  on 
entering  the  body,  the  light  is  said  to  be  refracted. 
When  the  light  which  enters  the  body  does  not  pass 
through  it,  the  light  is  then  said  to  be  absorbed.  Ab- 
sorbed light  is  generally  converted  into  heat ; but  it 
sometimes  renders  the  body  phosphorescent. 

252.  Diffusion  of  Light. — When  the  light  which 
falls  on  the  surface  of  a body  is  ihroivn  off  from  it  in 
all  directions.,  it  is  said  to  be  diffused.  Illuminated 
bodies  shine  by  means  of  dift'used  light,  which  they 
throAV  oft' in  all  directions. 

253.  Reflection  of  Light. — When  light  falls  on  the 
surface  of  a body,  and  is  thrown  off  from  it  at  equal 
and  opposite  angles  to.  that  at  which  it  struck  the  sur- 
face, it  is  said  to  be  reflected. 

Since  the  ether-waves,  which  are  the  cause  of  light,  are  elastic,  they 
are  reflected  from  the  surface  of  hard  bodies  in  the  same  manner  as 
any  other  elastic  body. 

254.  Laws  of  the  Reflection  of  Light.  — 1st.  The 

angle  of  reflection  is  eipaal  to  the  angle  of  incidence. 


LIGHT— ITS  NATURE  AND  SOURCES.  215 


Let  a ray  of  liglit,  A B,  Fig.  99,  fall  on  a reflecting 
surface  at  the  point 
B.  At  this  point 
draw  the  perpen- 
dicular, D B,  then 
A B D the  angle 
of  incidence,  and 
D B 6'  is  the  angle 
of  reflection. 

2d.  The  light  will 
he  reflected  in  the  same 
plane  as  that  in  which 
the  incident  ray  and  the  perpendicular  at  the  point  of  in- 
cidence lie. 

Thus,  if  the  incident  ray,  A B,  and  the  perpendicular,  D B,  lie  in 
the  plane  of  the  paper,  the  reflected  ray,  B C,  will  also  lie  in  the  plane 
of  the  paper,  and  not  above  or  below  it. 

255.  Amount  of  Light  Reflected. — The  amount  of 
light  reflected  at  any  surface  depends, 

1st.  On  the  kind  of  material  forming  the  surf ace,  and 
its  degree  of  polish. 

2d.  On  the  angle  at  which  the  light  strikes  the  sur- 
face. 

Highly-polished  metals  and  glass  are  excellent  reflect- 
ors of  light.  Most  light  is  reflected  from  the  surface 
of  a transparent  substance,  like  glass  or  water,  when  the 
light  falls  obliquely  on  the  surface.  When  light  falls 
on  such  surfaces  nearly  at  right  angles  to  the  surface, 
most  of  the  light  passes  through  the  body.  When  the 
sun  is  nearly  overhead,  we  can  look  at  his  image  in  a 
water-surface  without  being  dazzled,  because  but  little 
of  the  light  is  reflected ; but  when  the  sun  is  nearly 
setting,  the  image  is  too  dazzling  to  be  looked  at  stead- 
ily. Most  light  is  -reflected  from  opaque  surfaces,  like 


216 


NATURAL  PHILOSOPHY. 


those  of  polished  metals,  when  the  light  falls  the  most 
directly  on  the  surface  — that  is,  at  right  angles  to  it. 

256.  How  Bodies  become  Visible.  — Only  those 
bodies  are  visible  which  throw  off  light  in  all  directions. 
Both  luminous  and  illumined  bodies  do  this,  and  hence 
are  visible.  Illumined  bodies  are  visible  on  account 
of  the  diffused  light  they  throw  off'.  A body  which 
regularly  reflects  light  cannot  be  seen.  A fine  2Dolished 
mirror  is  invisible,  and  is  often  mistaken  for  an  open 
doorway.  When  a mirror  is  tarnished  or  covered  with 
dust,  it  then  diff'uses  a part  of  the  light  which  falls  upon 
it,  and  thus  becomes  visible. 

A ray  of  light  is  invisible  unless  some  light  from  it 
enters  the  eye.  We  do  not  see  the  rays  which  pass 
from  the  stars  to  the  earth.  The  path  of  a raj'  through 
a dusty  room  is  visible  because  the  particles  of  dust 
scatter  or  diff'use  the  light. 

257.  Absorption  of  Light. — AYhen  the  ether-waves 
fall  on  a body,  and  are  neither  transmitted  through 
it  nor  are  thrown  off  from  its  surface,  they  pass  into 
the  body  and  are  absorbed.  Absorbed  light  generally 
causes  the  molecules  of  the  body  to  vibrate  so  tis  to 
jwoduce  heat.  It  sometimes  causes  the  molecules  to 
vibrate  rapidly  enough  to  produce  light,  when  the 
body  is  said  to  be  phosphorescent. 

When  a surface  absorbs  most  of  the  light  which 
falls  on  it,  it  appears  black  or  dark,  because  it  diffuses 
but  little  light.  No  surface  absorbs  all  light  which 
falls  on  it,  since  we  know  of  no  bodies  so  black  as  to 
be  invisible. 

258.  Phosphorescence. — Phosphorescent  bodies  are 
those  that,  when  exposed  to  a bright  light,  will  con- 
tinue to  shine  for  some  time  after  they  are  taken  into 


LIGHT— ITS  NATURE  AND  SOURCES.  217 


the  dark.  The  ether-waves  in  being  absorbed  im- 
part their  motion  to  the  molecules,  and  cause  them 
to  vibrate  so  as  to  give  out  light  or  become  luminous. 

The  term  phosphorescence  is  sometimes  also  applied  to  the  faint  light 
emitted  by  glow-worms,  fire-flies,  and  jelly-fish,  or  by  decaying  animal 
and  vegetable  substances.  This  is  quite  different  from  the  phosphores- 
cence just  described,  and  is  due  to  a slow  oxidation  of  a substance  pro- 
duced by  the  animal,  or  which  results  from  the  decomposition  of  decay- 
ing animal  or  vegetable  matter. 


259.  Refraction  of  Light. — When  light  passes  from 
one  transparent  substance  to  another,  it  is  bent  out  of 
its  straight  course,  or  refracted.,  as  it  enters  the  other 
substance.  While  passing  through  this  substance,  how- 
ever, it  moves  in  straight  lines. 

Thus,  if  a ray  of  light,  D A,  Fig.  100,  pass  through  the 
air  and  fall  on  a water-sur- 
face, S B,  at  the  point  A, 
a part  will  be  reflected  in 
the  direction  A E ; but 
the  part  which  enters  the 
Avater  does  not  continue 
in  the  same  direction.,  A F, 
it  had  while  in  the  air.,  but 
is  bent  at  the  point  where 
it  strikes  the  surface..  A, 
and  takes  the  direction  A G. 

If  the  light  is  refracted,  it  must  either  pass  on  the 
side  of  A F,  which  is  farther  from  the  perpendicular, 
A H,  or  on  the  side  AA'hich  is  nearer  it.  When  light 
passes  from  a rare  to  a dense  medium,  as  from  air  into 
Avater  or  glass,  it  is  bent  towards  the  perpendicular ; when 
it  passes  from  a dense  to  a rare  medium,  as  from  Avater 
or  glass  into  air,  it  is  bent  from  the  perpendicular. 

19 


218 


NATURAL  PHILOSOPHY. 


260.  Laws  of  Refraction  of  Light. — 1st.  The  inei 
dent  ray,  the  perpendicular  at  the  point  of  incidence,  and 
the  refracted  ray,  all  lie  in  the  same  pilane. 

2d.  Between  the  same  ivjo  media,  the  sine  of  the  anyle 
of  refraction  bears  a constant  ratio  to  the  sine  of  the  anyle 
of  incidence,  whatever  may  he  the  anyle  of  incidence. 

In  Fig.  101  let  a circle  be  described  about  the  point  of  incidence,  I, 


by  the  radius,  D I,  and  let  lines,  D N and  P S, 
be  drawn  from  the  ends  of  the  radii,  ID  and 
I S,  at  right  angles  to  N I P,  the  perpen- 
dicular at  the  point  of  incidence  ; then  these 
lines,  D N and  P S,  will  be  respectively  the 
sines  oi  D I N and  SIP,  the  angles  of  inci- 
dence and  refraction.  Now,  between  the  same 
two  media,  the  sines  of  the  angles  of  incidence 
and  refraction  always  hear  the  same  ratio  to 
each  other,  no  matter  what  may  be  the  angle 


Fig.  101. 


Index  of  Refraction.  incidence. 

This  ratio  is  called  the  index  of  refraction,  and  may  he  represented  as 


D V 

follows,  viz. ; the  index  of  refraction  ==-p~^.  The  index  of  refraction 


varies  for  different  media,  hut  is  constant  for  the  same  two  media. 

The  refraction  ivhich  occurs  ivhen  light  passes  from 
air  to  water  may  be  shown  as  follows  : 

Experiment. — Place  a coin,  a.  in  the  bottom  of  an  empty  howl,  ..4, 


Fig.  102,  and  stand  in  such  a position 


that  the  coin  is  just  invisible,  as  at  c. 
Let  some  one  quietly  pour  clear  water 
-.X  into  the  basin,  and  the  ray  of  light  which 

just  passes  the  edge  of  the  basin  will 


be  bent  as  it  passes  out  of  the  water, 
and,  taking  the  direction  h c,  will 
enter  the  eye  of  the  observer,  who 


Fig.  102.— An  Effect  of  Refraction,  will  see  the  coin  in  the  position  a'. 


Caution.  — Place  the  eye  so  as  just  to 


see  the  coin  over  the  edge  of  the  empty  basin ; then  move  the  head  back 
until  the  coin  just  disappears;  this  will  be  the  best  position  in  which 
to  stand. 


LIGHT— ITS  NATURE  AND  SOURCES.  219 


261.  Effects  Caused  by  Refraction.  — To  an  ob- 

sorver  at  c,  the  coin  appears  to  be  raised  from  its  true 
position,  a to  a'.  This  effect  is  produced  when  Ave  are 
looking  at  things  in  water,  and  is  caused  by  refraction. 

In  Fig.  102,  the  light  is  represented  as  coming  from 
one  point  of  the  coin.  The  diA^erging  pencil  of  light, 
which  enters  the  eye,  appears  to  diverge  from  the  point 
a',  and  consequently  the  eye  sees  the  image  of  this  point 
of  the  coin  at  a',  and  not  at  a.  The  coin  and  bottom 
of  the  vessel  appear  raised,  and  the  water  appears  less 
deep  than  it  really  is,  a circumstance  Avhich  often  causes 
errors  of  judgment  in  entering  the  Avater. 

A stick  partly  immersed  in  water  appears  bent  at  the 
surface,  from  the  refraction  of  the  hght  at  that  point. 

262.  Intensity  of  Light.  — The  amount  of  light  re- 
ceived by  any  surface  is  smaller  the  greater  its  distance 
from  the  luminous  source,  or  in  other  words,  the  inten- 
sity of  the  light  it  receives  decreases  as  the  distance 
increases.  The  rate  of  this  decrease  may  be  expressed 
as  follows,  aTz.  : 

The  intensity  of  the  light  received  hy  ciny  surface  is 
inversely  proportional  to  the  square  of  its  distance  from 
j the  luminous  source. 

L If  an  opaque  body  be  placed  in  the  path  of  a beam 
j;  of  parallel  rays,  the  intensity  of  the  light  it  receives 
will  be  sensibly  the  same  at  all  distances ; but  when 
I illumined  by  a diverging  pencil,  such  as  is  given  off 
; from  all  luminous  points,  the  intensity  decreases  as  the 
square  of  the  distance  from  the  source. 

; Let  A,  in  Fig.  103,  be  a luminous  point,  and  A an 
opaque  screen  one  inch  square ; then  A Avill  receiA^e  a 
1 certain  amount  of  light,  depending  on  its  size  and  its 
1 distance  from  S.  Let  its  distance  from  S be  one  foot, 

i 

J 

\ 


220 


NATURAL  PHILOSOPHY. 


then,  if  a screen  be  placed  at  i?,  two  feet  from  /S',  it  will 
receive  a shadow  four  times  as  large  as  ; at  three  feet 
from  *S',  or  at  (7,  it  will  receive  a shadow 
nine  times  as  large  as  A,  and  at  four  feet, 
or  at  D,  sixteen  times  as  large.  If,  now, 
the  opaque  body.  A,  be  placed  at  £,  the 
quantity  of  light  it  vdll  receive  will  be  ' 
but  one-fourth  that  which  it  received  at  | 
A,  and  consequently  the  intensity  of  the  | 
light  it  receives  vdll  be  but  one-fourth  its  ^ 
intensity  at  J. ; at  C the  intensity  of  the  ^ 
light  will  be  but  one-ninth,  and  at  D but 
one-sixteenth. 

If  the  eye  be  placed  at  A,  the  amount 
of  light  which  it  receives  will  be  limited 
by  the  size  of  the  pupil  or  the  opening 
through  which  light  enters  the  eye.  IVhen  ■ 
placed  at  .S,  the  size  of  this  opening  re- 
maining sensibly  the  same,  the  amount  of 
light  the  eye  receives  is  but  one-fburth  of 
that  received  at  A. 

263.  Photometers. — -lYe  can  measure  the  intensity 
of  different  lights  by  means  of  instruments  called  j)ko- 
tometers. 

If  a faint  grease  spot  be  made  in  a sheet  of  paper,  it 
becomes  visible  when  held  between  the  eye  and  a source 
of  light,  because  more  light  comes  through  where  the 
paper  is  greased  than  elsewhere.  If  it  he  held  between 
two  sources  of  light.,  so  as  to  be  equally  ilhrmined,  the 
grease  spot  will  disaqjpear,  since  it  will  then  be  no 
brighter  than  the  rest  of  the  paper ; but  if  it  be  moved 
towards  either  light,  the  spot  will  be  more  illumined 
than  the  rest  of  the  paper,  and  will  again  appear.  This 


Pig.  103. 
The  Intensity 
of  Light. 


LIGHT— ITS  NATURE  AND  SOURCES.  221 


is  the  principle  of  Bunsen's  Photometer.^  in  which  a faint 
grease  spot  is  made  in  a sheet  of  paper  supported  in  a 
frame.  The  paper  is  placed  between  two  lights,  and 
moved  backwards  and  forwards  until  a position  is  ob- 
tained at  which  the  spot  disapj^ears.  Call  the  two  lights 
A and  and  suppose  the  screen  be  one  foot  from  A 
and  two  feet  from  .6,  then  B has  four  times  the  inten- 
sity of  A. 

261:.  Images  Formed  by  Small  Openings. — If  the 

rays  of  light  from  brightly  illumined  objects  be  allowed 
to  come  through  a small  opening  into  a dark  room  in 
which  a white  screen,  ri,  Fig.  101,  is  placed  in  front 


Fig.  104,  — Images  Formed  ty"  Small  Apeitnies. 


of  the  opening,  they  will  form  on  the  screen  an  accurate 
image  of  the  objects  from  which  they  came.  The  image 
Avill  have  the  same  colors  as  the  object,  but  will  be  in- 
verted, that  is,  turned  upside  down,  and  from  right  to 
left.  The  size  of  the  image  will  depend  on  the  distance 
of  the  screen  from  the  opening. 

The  diffused  light,  coming  from  all  points  of  the  ob- 
ject, enters  the  opening  and  produces  on  the  screen  an 
19* 


222 


NATURAL  PHILOSOPHY. 


exact  representation  of  tliose  parts  from  wliicli  it  came. 
As,  liowever,  the  rays  cross  at  the  opening,  the  light 
from  the  top  parts  of  the  object  will  be  received  on  the 
lower  parts  of  the  screen,  and  those  of  the  lower  parts 
of  the  object  on  the  top  parts  of  the  screen;  the  image 
must  therefore  be  inverted. 

Experiment.  — Allow  the  sunlight  to  pass  through  a hole  in  the 
shutter  of  a darkened  room,  and  let  the  image  fall  on  a piece  of  white 
paper,  held  at  right  angles  to  the  direction  in  which  the  light  is  enter- 
ing the  room.  A round  disc  of  light  will  be  seen  on  the  paper.  This 
is  the  image  of  the  sun. 

Experiment.  — Paste  a flat,  smooth  piece  of  tin-foil  over  the  hole  in 
the  shutter,  and  punching  a hole  in  the  foil  with  a large  pin  or  needle, 
allow  the  diffused  light  from  the  trees,  houses,  or  other  objects  outside, 
to  fall  on  a screen  held  opposite  the  pin-hole.  An  inverted  image  of 
the  things  outside  will  be  seen  on  the  paper. 

Experiment. — •Unsolder  the  top  from  an  empty  tomato-can,  by  | 
holding  it  in  the  flame  of  a Bunsen  burner,  and  punch  a hole  in  the  i 
bottom  with  a nail.  Paste  a piece  of  tin-foil  over  the  nail-hole,  and  | 
make  a pin-hole  in  it.  Cover  the  open  end  of  the  can  with  a piece  of  | 
oiled  paper.  If  now  a lighted  candle  be  brought  near  the  pin-hole,  ' 

an  inverted  image  of  the  caudle  will  be  seen  on  the  oiled  paper.  , 

Caution.  — Round,  smooth-edged  holes  are  preferable.  Large  holes 
give  brighter,  but  less  distinct  and  sharp  images  than  small  holes. 

265.  Mirrors  and  Specula.  — A .liigbly-polished  ! 

body,  having  a regular  surface  and  capable  of  reflect-  | 
ing  most  of  the  light  which  falls  upon  it,  is  called  a • 

mirror  or  a spectdum.  A reflector  made  of  glass,  cov- 
ered on  the  back  with  some  good  reflecting  surface,  is 
called  a mirror ; a highly-polished  metallic  reflector  is 
called  a speculum.  Mirrors  or  specula  may  be  either 
plane  or  curved. 

266.  Images  Seen  in  Plane  Mirrors.  — M^hen  an 
object  is  placed  in  front  of  a plane  mirror,  an  image 
the  same  size  as  the  object  will  be  seen  as  far  back  of 
the  mirror  as  the  object  is  in  front  of  it. 


LIGHT— ITS  NATURE  AND  SOURCES.  223 


We  always  see  an  image  in  the  direction  in  wliicli 
a ray  of  light  coming 
from  the  object  enters 
the  eye.  If,  therefore, 
an  object,  such  as  a can- 
dle, A B.  Fig.  105,  he 
placed  before  the  plane 
mirror,  C B,  the  image 
will  be  seen  by  an  eye  at 
P,  as  though  it  were  at 
M B'  back  of  the  mir-  Pormed  by  Plane  Mirrors. 

ror.  Every  point  of  the  object,  as  sends  a cone  of 
rays  to  the  mirror,  a part  only  of  which,  however,  enters 
the  eye  after  reflection.  This  poiot  of  the  object  will 
appear  to  be  situated  back  of  the  mirror  wdiere  those 
rays  apparently  meet. 

It  can  be  proved  by  geometry  that  this  distance,  C fl.', 
back  of  the  mirror  is  eqnal  to  the  distance,  C A.,  of  the 
luminous  point  in  front  of  it.  The  image  appears  as 
far  back  of  the  glass  as  the  object  itself  is  in  front  of  it. 

The  image  which  appears  back  of  the  glass,  and 
which  is  formed  by  rays  which  do  not  come  directly 
from  the  object,  is  called  the  virtual  image.  If  the 
eye  looked  at  J.  P it  Avould  see  a real  image;  that 
is,  one  formed  by  rays  coming  straight  from  the  ob- 
ject. 

Plane  mirrors  cause  the  image  to  appear  inverted 
from  right  to  left. 

Experiment. — Write  on  a sheet  of  paper,  and  before  the  ink  dries 
press  a clean  piece  of  blotting  paper  on  the  writing,  and  on  removing 
the  blotter  it  will  have  a copy  of  the  writing,  inverted  from  right  to 
left,  just  as  a mirror  appears  to  invert  objects ; for,  hold  the  blotter  in 
front  of  a looking-glass,  and  the  writing  on  the  blotter  can  easily  be 
read  in  the  glass. 


B D B' 


224 


NATURAL  PHILOSOPHY. 


The  fact  that  the  eye  sees  an  image  in  the  direction 
in  which  the  rays  enter  it  can  be  amusingly  shown  as 
follows ; 


Experiment. — By  placing  four  small  pieces  of  looking-glass  at  a,  h, 

c,  and  d,  as  shown  in  Fig.  106,  a ray 
of  light  from  a distant  object  will, 
after  reflection  from  the  mirrors,  enter 
C the  eye  at  C,  in  the  same  direction  as 
that  in  which  it  came  from  the  ob- 
ject; the  eye,  therefore,  will  see  the 
object,  although  an  opaque  object, 
such  as  a brick,  be  held  at  B,  between 
the  eye  and  the  object,  thus  making 
it  appear  as  though  the  person  was  seeing  through  the  brick.  The 
mirrors  may  be  concealed  in  a suitably  shaped  box,  with  openings  at 
A and  C. 


Fig.  106.  — Looking  throngli  a Brick. 


When  any  object  is  placed  between  two  plane  mir-  1 
rors  inclined  at  any  angle  to  eacli  other,  a number  of 
images  will  be  seen,  which  Avill  be  greater  as  the  inch-  j 
nation  between  the  two  mirrors  is  less.  This  multi-  , 
plication  of  the  image  of  an  object  is  seen  in  the  kalei-  < 
doscope.  , 

Experiment.  — Place  two  looking-glasses,  or  pieces  of  glass,  at  any  i 
angle  with  each  other,  and  observe  the  images  of  an  object  placed  I 
between  them.  Now  change  the  inclination  of  the  mirrors,  and  note  j 
the  change  in  the  number  of  images.  ! 

267.  The  Visual  Angle. — The  rays  of  light  which  i 
come  from  the  extremities  of  an  object  meeting  at  the  i 
eye,  form  an  angle  called  the  visual  angle.  ^Ve  judge  ! 

^ of  the  size  of  an  oh-  ; 

ject  mainly  by  means  i 
of  the  Ausual  angle: 
the  larger  the  visual 
angle  the  larger  does  : 
the  object  appear.  Thus,  in  Fig.  107,  the  A'isual  . 
angle  under  Avhich  the  eye  sees  the  object  J.  J.  is  | 


Fig.  107.  — The  Visual  Angle. 


LIGHT— ITS  NATURE  AND  SOURCES.  225 

A 0 A.  If,  now,  tlie  object  be  carried  to  A'  A',  it  will 
then  be  seen  under  tlie  smaller  visual  angle  A'  0 A'.,  and 
■will,  therefore,  appear  smaller.  The  smaller  object,  (7(7, 
is  seen  under  the  same  visual  angle  as  A' and  there- 
fore appears  of  the  same  size. 

Any  cause  which  alters  the  value  of  the  visual  angle, 
changes  the  apparent  size  of  the  object. 

268.  Curved  Mirrors.  — Curved  mirrors  may  be  of 
a variety  of  forms.  We  will  consider  two  of  these 
forms,  "viz.,  concave  and  convex  mirrors.  Concave  mir- 
rors are  curved  like  the  inside  of  a watch  crystal.  Con- 
vex mirrors  are  curved  like  the  outside  of  the  crystal. 

When  rays  of  light  from  illumined  objects  enter  the 
eye  after  reflection  from  curved  mirrors,  their  direction 
is  generally  so  changed,  that  the  ■visual  angle  under 
which  the  eye  ■views  the  image,  is  different  from  that 
under  which  it  would  have  -viewed  the  object,  had  it 
observed  it  directly.  The  apparent  size  of  the  image, 
therefore,  is  different  from  the  apparent  size  of  the  object. 

Whenever  a number  of  rays  collect  at  a single  point, 
that  point  is  called  a focus. 

269.  Concave  Mirrors. — Parallel  rays  of  light  fall- 
ing directly  on  a concave  mirror,  collect,  after  reflec- 


Fig.  108. — Action  of  Concave  Mirror  on  a Beam  of  Light. 


tion,  at  a point  in  front  of  the  mirror.  This  point  is 
called  the  principal  focus  of  the  mirror,  and  is  situated 


226 


NATURAL  PHILOSOPHY. 


midway  between  the  centre  of  the  mirror  and  the  centre 
of  the  sphere  of  which  the  mirror  may  be  conceived  to 
be  a part.  Thus,  in  Fig.  108,  the  principal  focus  is 
shown  at  F,  midway  between  the  mirror  and  the  point 
(7,  called  the  centre  of  curvature. 

If  a luminous  point  be  placed  at  Z,  more  distant  from 
the  mirror  than  the  centre  of  curvature,  the  diverging 
rays  which  it  casts  on  the  mirror  Avill,  after  reflection, 
converge  to  a focus  at  S.  Conversely  if  the  luminous 
point  be  placed  at  S,  the  focus  vdll  be  at  L.  The  points 
L and  S are  called  respectively  the  longer  and  shorter 
conjugate  foci. 

If  the  source  of  light  be  placed  at  (9,  between  the 
principal  focus  and  the  mirror,  the  rays  vdll,  after  re- 
flection, appear  to  come  from  a point,  ZT,  back  of  the 
mirror,  called  the  virtual  focus. 

270.  Images  Formed  by  Concave  Mirrors.  — If 

an  object  be  placed  before  a concave  mirror,  between 
tlie  mirror  and  the  principal  focus,  the  image  vdll  ap- 
pear loch  of  the  mirror  erect  and  larger  than  the  object. 

The  manner  in  which  the 
visual  angle  is  changed  by 
reflection,  is  seen  in  Fig. 
109,  in  which  a person  is 
represented  as  looking  at 
his  magnified  image  in  a 
concave  mirror.  The  rays 
Fig.  109.  of  light  coming  from  anv 

Virtual  Image  in  Concave  Mirror. 

fall  on  the  mirror,  being  reflected  from  it  as  shovTi 
by  the  arrow,  and  enter  the  eye  of  the  observer  as 
though  they  came  from  a point,  o',  back  of  the  mirror, 
so,  also,  those  coming  from  the  point  b are  so  changed 


LIGHT— ITS  NATURE  AND  SOURCES.  227 


in  tlieir  direction  by  reflection  as  to  appear  to  come 
from  the  point  b'.  The  observer,  therefore,  sees  an  en- 
larged erect  image  at  a'  V. 

If  the  object,  such  as  a candle,  be  placed  before  a 
concave  mirror,  at 
a shorter  conju- 
gate focus,  an  in- 
verted and  mag- 
nified image  will 
be  seen  at  the 
longer  conjugate 
focus,  as  shown  in 
Fig.  110  at  : but 
if  the  object  be 
placed  at  the 
longer  conjugate  focus,  the  image  will  be  inverted  and 
smaller  than  the  object,  and  will  be  seen  at  the  shorter 
conjugate  focus. 

The  foci  of  convex  mirrors  are  similar  to  those  of 
concave  mirrors,  but  are  formed  on  opposite  sides  of 
the  mirror  to  what  they  are  formed  in  concave  mirrors. 

J Syllabus. 

|.  Light  is  caused  by  a vibratory  motion  of  the  luminiferous  ether. 

I When  the  motion  imparted  to  the  luminiferous  ether  by  the  molecules 
; of  heated  bodies  is  sufficiently  rapid,  it  becomes  visible  as  light. 

[;  Invisible  ether-motion  constitutes,  in  most  cases,  radiant  heat.  The 
I visible  ether-motion  also  possesses  heating  powers.  Neither  the  visible 
nor  the  invisible  ether-motion  is  to  be  confounded  with  temperature, 
which,  in  every  case,  is  caused  by  the  vibrations  of  the  molecules  of 
; the  body,  and  never  by  the  vibration  of  the  ether. 

I The  principal  sources  of  light  are  the  sun,  stars,  chemical  combina- 
I tion,  and  electricity. 

i A body  which  gives  off  the  light  it  produces,  is  called  a luminous 


228 


NATURAL  PHILOSOPHY. 


body.  One  which  gives  off  the  light  it  receives  from  other  bodies,  is 
called  an  illuminated  body. 

Transparent  bodies  allow  us  to  clearly  see  other  bodies  through 
them.  Translucent  bodies  allow  light  to  pass  through  them,  but  will 
not  allow  other  bodies  to  be  seen  through  them.  Opaque  bodies  do 
not  allow  any  light  to  pass  through  them. 

A single  line  of  light  is  called  a ray;  a number  of  parallel  rays  is 
called  a beam  ; a cone  of  light  is  called  a pencil ; pencils  may  be  either 
converging  or  diverging. 

Light  moves  in  straight  lines.  When  it  meets  an  opaque  body,  a 
shadow  is  formed  back  of  the  body.  The  umbra  or  complete  shadow 
receives  no  light;  the  penumbra  or  partial  shadow  is  partially  illu- 
mined. The  velocity  of  light  is  about  185,000  miles  per  second. 

When  light  falls  on  the  surface  of  a body,  it  is  either  thrown  off  from 
the  surface  or  it  enters  the  body.  That  which  is  thrown  off  from  the 
surface  is  said  to  be  diffused  when  it  passes  off  in  all  directions ; but  to 
be  reflected  when  it  passes  off  in  only  certain  directions. 

The  light  which  enters  the  body  either  passes  through  it,  and  is 
bent  out  of  its  course  or  refracted  on  entering  the  body,  or  it  is  absorbed 
by  the  body.  When  absorbed  it  causes  heat,  or  renders  the  body  phos- 
phorescent. 

The  laws  for  the  reflection  of  light  are  the  angle  of  incidence  is  equal 
to  the  angle  of  reflection;  and  the  incident  ray,  the  perpendicular 
at  the  point  of  incidence,  and  the  reflected  ray,  all  lie  in  the  same 
plane. 

The  amount  of  light  reflected  at  any  surface  depends  on  the  kind  of 
material  forming  the  surface,  its  degree  of  polish,  and  the  angle  at 
which  the  light  is  incident. 

Bodies  become  visible  by  means  of  the  light  which  they  give  off  in 
all  directions.  When  a ray  of  light  passes  from  a rare  to  a dense  me- 
dium, it  is  refracted  or  bent  out  of  its  original  course  towards  the  per- 
pendicular at  the  point  of  incidence;  if  it  come  from  a dense  into  a 
rare  medium,  it  is  refracted  from  the  perpendicular. 

Whatever  be  the  angle  of  incidence,  the  angle  of  refraction  bears  a 
constant  ratio  to  the  angle  of  incidence,  provided  the  light  is  passing 
through  the  same  two  media.  The  incident  ray,  the  perpendicular 
at  the  point  of  incidence,  and  the  refracted  ray  all  lie  in  the  same 
plane. 

The  refraction  of  light,  in  passing  from  water  into  the  air,  causes 
the  water  to  appear  less  deep  than  it  actually  is. 

The  intensity  of  light  decreases  as  the  square  of  the  distance  from 
the  source. 


QUESTIONS  FOR  REVIEW. 


229 


The  relative  intensity  of  different  lights  is  measured  by  means  of 
instruments  called  photometers. 

Rays  of  light  passing  through  a small  aperture  in  the  wall  of  a 
darkened  room  form  an  inverted  image  of  the  object  from  which  they 
came. 

Mirrors  and  specula  are  polished  bodies  with  regular  surfaces,  that 
are  capable  of  reflecting  most  of  the  light  which  falls  on  them.  They 
are  either  plane  or  curved. 

Images  formed  by  plane  mirrors  are  of  the  same  size  as  the  objects, 
and  appear  to  be  as  far  back  of  the  mirror  as  the  object  is  in  front 
of  it. 

The  visual  angle  is  the  angle  formed  by  the  rays  of  light  from  the 
extremities  of  an  object  meeting  at  the  eye.  The  apparent  size  of  an 
object  depends  on  the  visual  angle  under  which  it  is  seen.  Curved 
mirrors,  by  altering  the  visual  angle  under  which  objects  are  seen, 
cause  them  to  appear  either  larger  or  smaller  than  they  actually  are. 

An  object  placed  before  a concave  mirror,  between  the  principal 
focus  and  the  mirror,  appears  enlarged,  erect,  and  back  of  the  mirror; 
if  placed  at  either  conjugate  focus,  it  is  seen  at  the  other  focus,  in- 
verted. 

Questions  for  Review. 

By  'what  is  light  caused  ? How  does  it  differ  from  heat  ? 

Name  the  principal  sources  of  light.  Distinguish  between  a lumi- 
nous and  an  illumined  body. 

Define  transparency,  translucency,  and  opacity.  Are  any  of  the 
metals  transparent? 

Distinguish  between  a ray,  a pencil,  and  a beam.  What  two  kinds 
of  pencils  are  there  ? 

How  can  you  prove  that  light  moves  in  straight  lines  ? 

How  are  shadows  caused?  Define  umbra  and  penumbra.  Upon 
what  does  the  shape  of  a shadow  depend?  Upon  what  does  its  size 
depend  ? Describe  any  experiment  in  shadows. 

What  is  the  velocity  of  light?  Upon  what  does  the  amount  of  light 
reflected  from  any  surface  depend  ? 

Define  diffusion,  reflection,  refraction,  and  absorption.  State  the 
laws  for  the  reflection  of  light. 

How  do  bodies  become  visible  ? When  only  are  rays  of  light  visible  ? 

Is  there  any  substance  known  which  can  absorb  all  the  light  which 
falls  on  it  ? 

20 


230 


NA  TURAL  PHIL  OSO PHY. 


In  wliat  two  different  senses  is  the  word  phosphorescence  used? 

What  do  you  understand  by  the  refraction  of  light  ? When  is  the 
ray  bent  towards  and  when  is  it  bent  away  from  the  perpendicular 
at  the  point  of  incidence  ? State  the  laws  for  the  refraction  of  light. 
Why  does  clear  water  appear  less  deep  than  it  really  is  ? 

Why  should  the  intensity  of  light  decrease  as  the  square  of  the  dis- 
tance ? Describe  Bunsen’s  photometer. 

Why  are  the  images  formed  by  light  passing  through  small  aper- 
tures inverted?  Describe  any  experiments  proving  that  the  images 
are  inverted. 

Distinguish  between  mirrors  and  specula. 

In  wha.t  direction  is  an  object  seen  by  the  eye  ? Describe  the  experi- 
ment of  seeing  through  a brick. 

How  far  back  of  a plane  mirror  does  the  image  of  an  object  appear? 

What  effect  is  produced  by  placing  an  object  between  two  inclined 
plane  mirrors  ? 

Define  visual  angle.  What  effect  has  the  visual  angle  on  the  ap- 
parent size  of  the  object? 

What  two  kinds  of  curved  mirrors  are  there? 

Define  principal  focus,  conjugate  foci,  and  virtual  focus  of  a concave 
mirror. 

Describe  the  position  of  the  image  of  an  object  placed  before  a con- 
cave mirror,  between  the  principal  focus  and  the  mirror ; when  placed 
at  the  longer  conjugate  focus ; at  the  shorter  conjugate  focus. 


CHAPTER  11. 


LENSES. —OPTICAL  INSTRUMENTS  AND 
VISION. 


271.  Effect  of  Refraction  on  Apparent  Direction. — 

When  rays  of  light  pass  through  a prism,  the  refraction 
they  undergo,  both 
on  entering  and  leav- 
ing the  prism,  causes 
them  to  emerge  con- 
siderably out  of  their 
original  direction. 

If  the  rays  of  light 
from  a candle  pass 
through  a prism  and 
enter  the  eye  of  an 

observer  placed  as  Fig.  lll.  — An  Effect  of  Eefraotion. 

shown  in  Fig.  Ill,  the  candle  will  appear  out  of  its 
real  position. 


272.  Lenses. — Lenses  are  generally  made  of  pieces 
of  transparent  glass  bounded  by  two  surfaces,  both  of 
which  are  curved ; or  one  of  which  is  curved  and  the 
other  plane. 

Eays  of  light  on  entering  and  leaving  a lens,  are 
refracted  or  bent  out  of  their  course.  Objects  seen 
through  lenses  will,  therefore,  as  in  the  case  of  a prism, 

231 


232 


NATURAL  PHILOSOPHY. 


appear  out  of  their  true  position.  The  amount  of  the 
displacement  of  an  image  depends  on  the  shape  of  the 
lens,  and  the  kind  of  material  of  which  it  is  made. 

273.  Forms  of  Lenses.  — Lenses  may  he  divided 
into  two  classes,  viz.,  converyimj  and  diveryiny. 

Converyiny  lenses  cause  the  diverging  rays  which 
come  from  a luminous  point  to  converge  or  collect  at 
one  point,  after  passing  out  of  the  lens. 

Diveryiny  lenses  cause  the  rays  from  a luminous 
point,  after  passing  through  the  lens,  to  diverge  as 
though  coming  from  some  other  luminous  point. 

The  luminous  point  may  be  so  placed  before  either 
converging  or  diverging  lenses  as  to  cause  the  rays  to 
emerge  from  the  lens  parallel  to  each  other. 

The  three  forms  of  converging  lenses  are  sho^vn  at 
A B and  (7,  Fig.  112,  and  the  three  forms  of 
diverging  lenses  at  D E and  F,  Fig.  113.  It 
will  be  noticed  that  the  converyiny  lenses  are 
all  thicker  in  the  middle  than  at  the  edyes.^ 
while  the  diveryiny  lenses  are  thinner  in  the 
middle  than  at  the  edyes. 

These  lenses  are  named  as  follows : .4  is  a double 
convex  leris,  or  more  frequently  a convex  lens ; £ is  a, 
plano-convex  lens ; (7  is  a converging  concavo-convex  lens, 
or  sometimes  a meniscus;  Z>  is  a double  concave  lens,  or 
more  frequently  a concave  lens;  H is  a plano-concave 
lens ; and  F a diverging  concavo-convex  lens. 


ABC 
Fig.  112. 
Converging 
Lenses. 


D E 
Pig.  113. 
Diverging 
Lenses. 

light  collect. 


274.  Foci  of  Lenses.  — The  foci  of 
lenses  are  the  points  at  which  the  rays  of 


In  what  is  about  to  be  said  concerning  the  foci  of  lenses,  it  must  be 
distinctly  understood  that  the  curvature  of  the  opposite  faces,  when 
both  are  curved,  is  supposed  to  be  the  same,  and  that  the  values  given 
for  the  foci  are  only  approximately  true  for  ordinary  glass.  The  kind 
of  glass  used  is  that  commonly  employed  in  making  small  lenses. 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  233 


The  principal  focus  of  a lens  is  its  focus  for  parallel 
rays.  The  principal  focus  of  a 
convex  lens  is  situated  at  about 
the  centre  of  curvature  of  the 
face  at  which  the  light  is  in- 
cident. Thus,  in  the  lens  shown 
in  Fig.  lid,  the  principal  focus 
is  shown  at  F.  The  principal  focus  of  a concave  lens 
is  situated  at  about  the  centre  of  curvature  of  the  face, 
at  which  the  light  is  incident.  Thus,  in  the  lens  shown 


Fig.  114,  — Principal  Focns 
of  Convez  Lens. 


in  Fig.  115,  the  principal 
focus  is  at  F.  The  con- 
cave lens  causes  the  light, 
after  passing  out  of  the 
lens,  to  appear  to  diverge.  Principal  Focus  of  Concave  Lens. 

as  though  it  came  from  the  point  F. 

The  Conjugate  Foci.  — If  a luminous  point  be  placed 
before  a convex  lens,  at  a distance  from  it  equal  to  twice 
its  radius  of  curvature,  the  rays,  after  emerging  from 
the  lens,  will  be  converged  to  a point  at  an  equal  dis- 
tance on  the  opposite  side  of  the  lens.  But  if  the  lu- 
minous point  be  situated  farther  from  the  lens  than 
twice  the  radius  of  curvature,  as  at  (7,  Fig.^116,  the  rays 
will  collect  at 

a focus,  C",  on 

' C' 

the  other  side 
of  the  lens, 
somewhere  Fed. 

between  the  centre  of  curvature  of  the  lens  and  a point 
situated  at  twice  the  distance  of  the  centre  of  curvature 
in  the  same  side.  These  foci,  G and  O',  are  called  re- 
spectively the  longer  and  shorter  conjugate  foci. 

The  Virtual  Focus.  — If  a luminous  point  be  placed 
before  a convex  lens,  at  0,  Fig.  117,  somewhere  between 
20* 


Fig.  115. 


234 


NATURAL  PHILOSOPHY. 


the  principal  focus  and  the  lens,  the  rays,  after  emerg- 
ing from  the  lens,  will  diverge  as  though  coming  from 

a point,  F,  on  the  same 
side  of  the  lens  as  the  [ 
luminous  point,  and  at  a | 
distance  from  it  greater  | 
than  the  distance  of  its  ; 
principal  focus.  This  is  called  the  virtual  focus. 

The  foci  of  concave  lenses  are  all  virtua  l.  ! 

275.  Images  Formed  by  Lenses.  — Whenever  the  \ 
visual  angle  under  which  the  eye  views  an  image  formed 
hy  a lens.!  is  different  from  that  under  which  the  eye  would  '• 
view  the  object  directly.^  the  apparent  size  of  the  object  is  ' 
different  from  its  real  size. 

If  an  object,  Fig.  118,  be  held  between  the  prin- 
cipal focus  and  the  lens,  an  eye  placed  on  the  other  side  ' 
A of  the  lens  will  see 

^ an  erect  and  masni- 

fied  image,  apparent- 
^ ly  situated  farther 

\J  from  the  lens  than 

Fig.  118. —Virtual  Image  of  Convex  Lens.  the  objcct. 

If  we  examine  any  object  with  a magnifying-glass,  we 
will  find,  if  we  move  the  object  towards  and  from  the 
lens,  \vithout  ever  taking  it  farther  from  the  lens  than  the 
principal  focus,  that  the  size  of  the  image  i\dll  vary,  but 
that  the  image  ivill  be  most  distinct  when  the  object  is  i 
held  at  a certain  distance  from  the  lens.  It  can  be  shown 
that  this  position  lies  very  near  the  principal,  focus.  The 
image  will  then  appear  at  the  distance  at  which  the  eye 
sees  objects  the  most  distinctly,  and  which  is  therefore 
called  the  limit  of  distinct  vision.  The  limit  of  distinct 
vision  varies  with  different  persons,  and  therefore  each 


Fig.  117.  —Virtual  Focus. 


[ 


I 

j 


i 

I 

I 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  235 

person  must  hold  the  glass  at  such  a distance  from  the 
object  that  the  image  formed  shall  be  at  his  limit  of  dis- 
tinct vision;  that  is,  he  must  focus  the  glass  to  suit  his  eyes. 


276.  Images  at  Conjugate  Foci.  — If  an  object 

be  placed  before  a 
convex  lens,  any- 
where between  the 
principal  focus  and 
twice  the  principal 
focus,  — that  is,  at 
the  shorter  conju- 
gate focus,— an  en-  . conjugate  Focus. 

larged  and  inverted 

image  will  be  formed  at  its  longer  conjugate  focus  ; but 
if  the  object  be  placed  at  the  longer  conjugate  focus, 
an  inverted  and  diminished  image  will  be  formed  at 
the  shorter  conjugate  focus.  In  Fig.  119,  the  object, 
A B,  is  placed  at  the  longer  conjugate  focus  of  the 
convex  lens,  0,  and  an  inverted  image,  smaller  than  the 
object,  is  seen  at  A'  B' . As  this  image  is  real,  it  may 
be  received  on  a 
screen,  as  shown. 

277.  The  Eye. 

— The  human 
eye  consists  of  a 
nearly  spherical 
chamber,  dark- 
ened on  the  inside 
and  provided 
withtwoopenings 

— one  in  trout  ® ■' 

for  the  admission  of  light,  and  one  at  the  rear.  Fig. 
120,  for  the  admission  of  the  nerve  called  the  ogotic 


236 


NATURAL  PHILOSOPHY. 


nerve^  which  conveys  the  imj^ressions  of  light  to  the 
brain,  and  thus  enables  us  to  see.  On  its  entrance  to 
the  chamber  of  the  eye  tTie  optic  nerve  is  spread  out 
into  a delicate  net-work  of  nerve  filaments,  called  the 
retina.,  seen  in  the  figure  at  K.  The  retina  acts  as  a 
curtain,  or  screen,  to  receive  the  image  formed  by  the 
lenses  of  the  eye.  At  the  opening  in  the  front  of  the 
eye  there  is  a transparent  substance,  called  the  cornea, 
seen  at  A,  more  convex  than  the  ball  of  the  eye.  Be- 
hind the  cornea,  and  forming  the  colored  part  of  the 
eye,  is  a circular  curtain,  called  the  iris,  seen  at  D.  A 
circular  aperture  in  the  middle  of  the  iris,  C,  called  the 
pupil,  forms  the  opening  through 'which  light  enters 
the  eye.  Immediately  back  of  the  iris  is  a convex 
lens,  E,  called  the  crystalline  lens.  The  space,  B,  be- 
tween the  cornea  and  the  crystalline  lens  is  filled  with 
a liquid  called  the  aqueous  humor.  The  large  cavity,  i 
L,  back  of  the  crystalline  lens  is  filled  with  a clear,  ! 
jelly-like  substance,  called  the  vitreous  humor.  ' 

All  these  transparent  portions  of  the  eye  act  lihe  one  con-  I 
vex  lens,  and  form  an  inverted  and  diminished  imaye  of  ] 
a distant  object  on  the  retina.  If  this  image  is  sufficient-  j 
ly  distinct,  is  properly  illuminated,  and  remains  long  { 
enough  on  the  retina,  the  object  is  seen  distinctly.  If  too  j 
much  light  enters  the  eye  through  the  pupil,  the  image 
is  not  seen  distinctly.  The  iris,  however,  is  so  affected 
by  the  light  that,  if  too  much  light  passes  through  the  1 
pupil,  the  pupil  contracts  or  grows  smaller ; while  if  | 
too  little  enters,  the  pupil  enlarges  and  allows  more  ; 
light  to  enter  the  eye.  ! 

Experiment. — Hang  a small  mirror  immediately  under  a gas-light. 
Look  in  the  mirror  at  the  image  of  your  eye,  and  note  the  size  of  \ 
the  pupil;  now  snddenly  turn  the  light  down,  leaving  only  sufficient  j 
light  to  see  the  image  of  the  eye.  The  pupil  will  now  he  seen  to  slowly 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  237 


dilate.  Turn  the  gas  on  bright  again,  and  the  pupil  will  he  seen  to 
contract. 

Caution.  — Do  not  cause  the  pupil  to  dilate  and  contract  too  sud- 
denly, as  weak  eyes  might  thereby  be  injured. 


278.  Near-sightedness  and  Long-sightedness. — 

Objects  are  not  seen  distinctly  unless  the  lenses  of  the 
eye  cause  the  images  to  fall  directly  on  the  retina. 

In  many  eyes,  the  lenses  converge  tlie  rays  so  much 
as  to  cause  the  images  of  distant  objects  to  be  formed 
in  front  of  tbe  retina.  Sucb  people  are  near-sighted, 
and  cannot  see  minute  objects  distinctly  witbout  bring- 
ing them  very  near  tbe  eye.  Near-sightedness  can  be 
partly  remedied  by  the  use  of  concave  spectacles. 

In  other  eyes,  tbe  lenses  of  tbe  eye  cause  tbe  images 
of  near  objects  to  fall  back  of  tbe  retina.  Sucb  people 
i are  long-sighted,  and  cannot  see  near  objects  distinctly; 
i thus,  in  reading,  tbe  book  must  be  beld  at  some  dis- 
I tance  in  front  of  tbe  eyes.  Long-sightedness  can  be 
\ partly  remedied  by  the  use  of  convex  spectacles. 


279.  Optical  Instruments.  — By  an  optical  instru- 
ment we  mean  any  combination  of  lenses,  or  of 
lenses  and  mirrors,  wbicb  will  enable  us  to  examine 
tbe  images  of  distant  or  near  objects.  In  optical  in- 
struments, a number  of  lenses  are 
generally  used,  yet  they  act,  as  far 
as  tbe  production  of  tbe  image  is 
concerned,  as  if  there  %vere  but  a 
single  lens  present ; for  example,  tbe 
simple  microscope,  the  pjhoto graphing 
camera,  the  magic  lantern,  o.nd  the 
camera  obscura. 

280.  The  Simple  Microscope. — Fig,  121, 

In  tbe  simple  microscope,  a single  The  Simple  Microscope. 

lens  or  combination  of  lenses  is  placed  as  shown  at 


238 


NATURAL  PHILOSOPHY. 


il,  in  Fig.  121.  The  object  to  be  examined  i.s  placed 
on  a stage,  (7,  at  a distance  from  the  lens,  rather  less 
than  its  principal  focus.  An  eye  placed  above  A sees 
an  enlarged  and  erect  image  of  the  object.  A screw, 
V,  is  provided  for  focussing. 

281.  The  Photographing  Camera.  — The  photo- 
graphing camera.  Fig.  122,  consists  of  a lens  placed  in 

a tube  at  A,  inserted  in 
the  camera-box,  (r.  The 
image  of  an  object,  for 
instance,a  person, placed 
in  front  of  the  tube.  A, 
at  the  longer  conjugate 
focus,  is  received  on  a 
screen  of  ground  glass, 
A,  as  an  inverted  and 
Fig,  122, -The  Photographic  Camera,  diminished  image.  This 

image  is  sharply  focussed  on  the.  screen  by  means  of  the 
screw,  D.  The  screen  is  then  removed  and  replaced 
by  a plate  covered  ivith  chemicals  sensitive  to  light. 
The  image  now  falls  on  the  sensitive  plate,  and  is  im- 
pressed upon  it  by  the  light  causing  certain  changes 
in  the  chemicals  on  its  surface. 


282.  The  Magic  Lantern.  — In  the  magic  lantera. 
Fig.  123,  a transparent  picture, 
A,  is  placed  in  an  inverted 
position,  before  a lens,  B.  at 
its  shorter  conjugate  focus,  and 
is  received  as  an  enlarged  and 
erect  image  on  a screen  placed 
in  front  of  the  lantern.  For 
The  Magic  Lantern.  the  purpose  of  stronglv  illu- 

mining the  picture,  a mirror  is  placed  at  J/,  back 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  239 


of  the  source  of  light,  L,  and  a lens,  (7,  called  the  con- 
denser, is  placed  in  front  of  the  light,  and  between  it 
and  the  picture. 

283.  The  Camera-Obscura.  — The  camera-obscura 
is  an  arrangement  by  which  a distinct  and  well  illu- 
mined image  of  any  object  can  be  thrown  on  a sheet 
of  paper  for  convenience  in  sketching.  A mirror,  i/. 
Fig.  12d,  is  placed  in  an  inclined  position  above  a lens,  L. 


Fig.  124.  — Camera  Obscnra. 


The  light  from  a distant  object  is  reflected  from  the  mir- 
ror to  the  lens,  which  forms  an  image  of  it  on  a sheet  of 
paper  placed  on  a table,  B,  at  a suitable  distance  below. 

Experiment. — Place  a small  convex  spectacle  lens  in  a suitable  hole 
in  the  top  of  a hox;  attach  a piece  of  looking-glass  directly  over  the 
lens,  and  inclined  at  an  angle  of  about  45°.  On  holding  a sheet  of 
paper  inside  the  box,  at  the  proper  distance  from  the  lens,  the  side  of  the 
box  having  been  removed  for  that  purpose,  an  image  of  any  object  in  front 
of  the  mirror,  as,  for  example,  a tree  or  person,  will  be  seen  on  the  paper. 

284.  The  Compound  Microscope.  — In  the  com- 
pound microscope  and  the  telescope,  at  least  two  sets  of 
lens  are  used. 


240 


NATURAL  PniLOSOPHY. 


In  the  compound  microscope,  Fig.  125,  instead  of 
looking  directly  at  the  object,  the  magnified  image 

formed  by  one  set 
of  lenses  is  viewed 
through  a second  leus, 
which  still  farther 
magnifies  it.  The 
lens  which  is  placed 
Pig.  125. -The  Compound  Microscope,  nearer  the  object,  is 
called  the  object-glass;  the  one  nearer  the  eye,  the  eye- 
lens.  The  object,  h a,  is  placed  before  the  object-glass, 
0,  at  its  shorter  conjugate  focus^  and  an  inverted  and 
enlarged  image  formed  by  it,  at  a'  V.  This  image,  a'  U, 
lies  nearer  to  the  eye-lens,  (7,  than  its  principal  focus, 
and  the  eye  at  D,  therefore,  sees  a greatly  enlarged 
image  at  A B. 

285.  The  Telescope.  — The  telescope,  Hke  the  mi- 
croscope, has  only  two  sets  of  lenses ; the  set  nearer  the 
object  is  called  the  object-glass,  and  that  nearer  the  eye 
the  eye-glass.  Telescopes  may  be  constructed  either 
with  both  object-  and  eye-lens  of  glass,  in  which  case 
they  are  called  refracting  telescopes ; or  a concave  mir- 
ror may  be  used  in  place  of  the  object-lens,  when  they 
are  called  reflecting  telescopes. 

In  the  refracting  telescope,  shown  in  Fig.  126,  0 is 

the  object-glass 
and  E the  eye- 
lens. 

Since  the  tel- 
escope is  u.'^ed 

forvievdnadis- 
Fig.  126.  — The  Eefracting  Telescope.  tautobject^  the 

object  is  at  the  longer  conjugate  focus.  Its  image,  there- 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  241 

fore,  seen  at  a h,  is  inverted  and  diminished.  As  in  the 
microscope,  this  image  falls  within  the  principal  focus 
of  the  eje-lens,  and  is  therefore  viewed  by  the  eye 
placed  at  as  a magnified  image.  A'  B' . 

286.  Reflecting  Telescopes  employ  a mirror  for 
the  object-glass  in- 
stead of  a lens.  In 
Fig.  127  we  have 
a representation  of 
Herschel’s  reflecting 

telescope,  in  which 
,1  • nr  c Fiff.  127.  — The  Eeflecting  Telescope, 

the  mirror,  ii,  forms  & r 

an  image,  a Zi,  of  a distant  object,  which,  viewed  by  the 
eye  at  0,  through  the  eye-lens,  A,  appears  as  a magni- 
fied image,  a'  V . 

The  penetrating  power  of  a telescope  is  the  distance 
at  which  it  can  collect  and  transmit  to  the  eye  suffi- 
cient light  from  a distant  object  to  enable  it  to  be  dis- 
tinctly seen. 

The  telescope  owes  its  great  penetrating  power  to 
the  size  of  the  object-glass.  In  the  unaided  eye,  the 
amount  of  light  which  can  enter  the  eye  from  a dis- 
tant object  is  limited  by  the  size  of  the  pupil ; but  by 
1 the  use  of  a telescope,  all  the  light  which  falls  on  the 
i object-glass  is  swept  into  the  eye.  The  penetrating 
I power  of  the  telescope  will,  therefore,  be  as  much 
[ greater  than  that  of  the  eye  as  the  area  of  the  object- 
glass  is  greater  than  the  area  of  the  pupil  of  the  eye. 
In  a large  reflecting  telescope,  made  by  an  Englishman 
named  Lord  Eosse,  the  area  of  the  concave  mirror, 

[ which  acted  as  the  object-glass,  was  518,400  times 
1 greater  than  the  pupil  of  the  eye.  This  instrument 
i collected,  even  after  allowing  that  half  the  light  was 
i 21  Q 


242  NATURAL  PHILOSOPHY.  3 

lost  by  reflection,  rather  more  than  250,000  times  as 
much  light  as  the  unassisted  eye. 

287.  Dispersion  of  Light  — The  Spectrum. — We 

have  seen  that  sounds  of  a single  pitch  are  seldom 
pure,  being  always  accompanied  by  a number  of  sounds 
of  some  other  pitch  : the  same  is  true  of  light.  Sun- 
light, though  apparently  pure, — that  is,  of  one  color, — 
in  fact  contains  a very  great  number  of  different-col- 
ored lights.  Idle  different  colors  that  are  present  in 
sunlight  may  be  separated  from  one  another  so  as 
to  be  visible  as  separate  colors,  by  passing  the  light 
through  a prism. 

If  sunlight  coming  through  a narrow  slit,  in  the 

shutter  of  a dark 
room,  in  the  direc- 
tion -FA",  be  allowed 
to  pass  through  a 
prism,  P,  as  shown  in 
Fig.  128,  the  light 
will  not  only  be  bent 
out  of  its  course  by 
Fig.  128.— Formation  of  a Spectrnm  by  a Prism,  refraction,  but  it  will 
also  be  separated  into  a number  of  differently  colored 
rays,  which,  if  allowed  to  fall  on  a screen,  ivill  be  spread 
out  in  the  form  of  a brightly  colored  band,  R T , called 
a spectrum.  There  are  almost  an  infinite  variety  of 
colors  in  the  spectrum  of  sunlight ; but,  for  the  sake 
of  simplicity,  we  may  distinguish  seven  well-marked 
regions  of  color,  which  are  called  violet,  indigo,  blue, 
green,  yellow,  orange,  and  red.  The  order  of  these 
colors  maybe  easily  remembered  by  the  word  vthgijor, 
which  is  formed  by  putting  together  the  first  letter  in 
each  of  the  colors. 


LENSES— OPTICAL  INSTRUMENTS,  VISION.  243 


It  will  be  observed  in  Fig.  128  that  the  different 
colors  of  the  spectrum  are  turned  out  of  the  direction 
in  which  the  sunlight  was  moving,  in  different  degrees  ; 
thus,  the  red  is  the  least  and  the  violet  the  most  turned 
out  of  its  course;  that  is,  the  different  colors  of  the  spec- 
trum differ  in  their  refranrjihility. 

The  separation  of  light  into  different  colors,  by  its 
passage  through  a prism,  is  called  dispersion. 

The  different  colors  of  the  spectrum  differ  in  the 
lengths  of  the  ether-waves  which  cause  them.  The 
wave-lengths  of  the  red  are  the  longest,  and  those  of 
the  violet  the  shortest.  These  waves  are  so  exceed- 
ingly small  that  37,640  of  the  red,  or  59,750  of  the 
violet,  would  only  equal  one  inch  in  length ! 

288.  The  Re-combination  of  the  Colors  of  the 
Spectrum. — If  all  the  colors  of  the  solar  spectrum  be 
again  mixed  together,  they  will  produce  white  light 
like  that  of  the  sun.  This  may  be  done  by  causing 
; the  spectrum  to  fall  on  a convex  lens,  and  allowing  the 
light  which  passes  through  to  fall  on  a screen  placed 
at  the  focus.  The  spot  of  light  formed  on  this  screen 
, will  no  longer  be  colored,  but  will  be  pure  white. 

Experiment.  — Allow  a narrow  beam  of  light  entering  a darkened 
room  through  a hole  in  the  shutter  to  pass  through  one  of  the  glass. 
' prisms  taken  from  a chandelier.  The  spectrum  formed  may  be  received 
' on  a sheet  of  white  paper  held  near  the  prism. 

\ Experiment. — Cut  out  a disc  of  stiff  paste- 
i board,  and  paint  on  it,  in  the  spaces  shown  in 
^ Fig.  129,  seven  colors,  as  near  as  can  he  ob- 
' tained  to  those  seen  in  the  solar  spectrum  or 
in  the  rainbow.  Stick  a pin  through  the  centre 
I of  tlie  disc,  and  whirl  it  rapidly  around  with 
; the  fingers.  When  it  is  turning  fast  enough  the 
disc  will  appear  to  be  painted  grayish  white, 
from  the  mingling  of  the  different  colors. 

Caution.  — Observe  the  size  of  the  spaces  shown  in  the  figure. 


244 


NATURAL  PHILOSOPHY. 


289.  The  Cause  of  Color. — When  sunlight  falls 
on  any  colored  body,  as,  for  example,  on  a piece  of  red 
cloth,  all  the  colors  of  the  light  but  the  reds  are  ab- 
sorbed, and  these  colors  only  being  given  off  from  the 
cloth,  it  appears  red ; so  in  a green  leaf  all  the  colors 
but  the  greens  are  absorbed,  and  the  greens  only  given 
off.  The  colors  of  bodies,  then,  are  due  to  the  light 
which  falls  on  them.  In  the  dark,  no  body  has  color.  ! 

If  a body  of  any  color,  such,  for  example,  as  pure  j 
red,  be  illumined  by  light  which  contains  no  red,  then  ' 
the  body  will  appear  black.  Thus,  if  a piece  of  bright  i 
red  flannel  be  held  in  the  pure  green  light  of  the  solar  | 
spectrum,  it  will  appear  black.  j 

Experiment. — Eoll  a piece  of  lamp-wick  into  a loose  ball,  and  soak 
it  for  a few  moments  in  very  strong  solution  of  salt  in  w'ater.  Then  ; 
place  it  in  a saucer  and  pour  some  alcohol  (spirits  of  wdne)  over  it,  and 
set  fire  to  the  wick.  It  will  hum  with  a pure  yellow  light.  If,  now,  , 
different-colored  objects,  such  as  zephyrs,  cloths,  or  silks,  be  examined 
by  means  of  this  light,  in  an  otherwise  darkened  room,  they  will  all 
appear  to  have  lost  their  color,  except  those  which  were  yellow.  Now 
bring  a lighted  candle  near  any  of  the  colors,  and,  since  the  lighted 
candle  gives  off  light  of  all  colors,  the  color  of  the  fabrics  will  again 
appear. 

Caution. — Unless  the  room  can  be  darkened  very  thoroughly,  this  i 
experiment  should  be  tried  at  night. 

290.  The  Rainbow.  — When  sunlight  passes 
through  falling  rain-drops,  it  is  separated  into  its  : 
different  colors,  the  rain-drops  acting  like  prisms.  In  i 
entering  the  drops  the  light  is  reflected  from  the  sur- 
faces farthest  from  the  sun,  and  passes  out  separated  , 
into  its  prismatic  colors.  This  light  entering  the  eye  ! 
of  an  observer  standing  with  his  baek  to  the  sun, 
causes  him  to  see  a band  of  colors  called  the  rainbow. 
Eainbows  are  largest  when  the  sun  is  near  the  horizon,  : 
as  when  nearly  setting,  or  shortly  after  rising. 


SYLLABUS. 


245 


291.  Relations  of  Light  and  Radiant  Heat. — Both 
light  and  radiant  heat  are  effects  produced  by  one  and 
the  same  cause,  viz.,  hy  vibrations  or  waves  in  the  lumi- 
niferous ether,  and  differ  only  in  the  fact  that  the  vibra- 
tions which  'produce  light  are  more  rapid,  and  the  waves 
shorter  than  those  which  produce  radiant  heat. 

When  any  solid  body,  as,  for  example,  a piece  of 
platinum,  is  heated,  its  molecules  vibrate  backwards 
and  forwards,  and  by  imparting  their  motion  to  the 
ether  outside  of  them,  give  off’  or  radiate  heat.  As  the 
body  becomes  hotter  and  hotter,  its  molecules  vibrate 
more  and  more  rapidly,  and  give  off  shorter  and  shorter 
waves  to  the  ether  outside  the  body,  until  at  last  these 
ether-waves  thus  radiated  from  the  body  become  short 
enough  to  aff’ect  the  eye,  and  the  body  is  red  hot  and 
radiates  red  light,  the  color  produced  by  the  longest 
ether-waves. 

If  now  we  continue  heating  the  platinum,  and  exam- 
ine, by  a prism,  the  light  it  radiates,  we  will  find,  as  it 
grows  hotter,  that  the  next  color  given  off  will  be  the 
orange,  the  next  longest  wave-length,  and  then  the  yel- 
low, the  green,  the  blue,  the  indigo,  and  the  violet ; at 
this  moment  the  body  will  be  white  hot,  and,  like  the 
sun,  will  give  off'  all  the  colors  of  the  spectrum. 

Syllabus. 

Objects  seen  through  a prism  appear  out  of  their  real  position,  on 
account  of  the  light  being  refracted  on  entering  and  leaving  the  prism. 

Lenses  are  pieces  of  transparent  glass  bounded  by  two  surfaces, 
both  of  which  are  curved,  or  one  of  which  is  curved  and  the  other 
plane.  Lenses  are  either  converging  or  diverging.  The  converging 
lenses  are  the  doubly  convex,  the  plano-convex,  and  the  converg- 
ing concavo-convex.  The  diverging  lenses  are  the  double  concave, 
the  plano-concave,  and  the  diverging  concavo-convex. 

21* 


246 


NATURAL  PHILOSOPHY. 


The  foci  of  lenses  are  the  points  at  which  they  cause  the  rays  pass- 
ing through  them  to  collect. 

The  principal  focus  of  a double  convex  lens  is  situated  at  the  centre 
of  curvature  of  one  of  its  curved  faces.  The  shorter  conjugate  focus 
is  situated  between  the  principal  focus  and  a distance  twice  as  great 
as  that  of  the  centre  of  curvature.  The  longer  conjugate  focus  is 
situated  farther  from  the  lens  than  twice  its  centre  of  curvature. 

Whenever  the  visual  angle  under  which  the  eye  views  an  image 
formed  by  a lens,  is  different  from  that  under  which  the  eye  would 
view  the  object  directly,  the  apparent  size  of  the  object  is  different 
from  its  real  size. 

When  we  look  through  a convex  lens  at  an  object  placed  between  the 
lens  and  its  principal  focus,  an  erect  magnified  image  is  seen  on  the  same 
side  of  the  lens  as  the  object,  but  farther  from  it  than  the  object.  If 
the.  object  he  placed  at  the  shorter  conjugate  focus,  a magnified  and  in- 
verted image  will  appear  at  the  longer  conjugate  focus.  If  the  object 
be  placed  at  the  longer  conjugate  focus,  an  inverted  and  diminished 
image  will  appear  at  the  shorter  conjugate  focus. 

The  eye  consists  of  a dark  chamber  containing  a number  of  lenses, 
which  cause  images  of  objects  in  front  of  the  eye  to  fall  on  a screen. 
This  screen  is  at  the  baek  of  the  eye. 

The  cause  of  near-sightedness  is  the  too  great  converging  power  of 
the  lenses  of  the  eye.  The  cause  of  long-sightedness  is  their  too  feeble 
converging  power.  In  near-sightedness  the  image  falls  in  front  of  the 
retina,  and  in  long-sightedness  it  falls  back  of  the  retina.  Near-sight- 
edness may  be  partly  remedied  by  the  use  of  concave,  and  long  sight- 
edness by  the  use  of  convex  spectacles. 

The  simple  microscope  contains  a single  convex  lens  or  set  of  lenses. 
The  object  to  be  examined  is  placed  rather  nearer  to  the  lens  than  its 
principal  focus ; the  images  it  forms  are  erect. 

In  the  photographing  camera,  the  object  is  placed  before  a single 
convex  lens  or  set  of  lenses,  at  the  longer  conjugate  focus.  The  image 
is  received  by  a sensitive  plate  placed  at  the  shorter  conjugate  focus. 

In  the  magic  lantern,  a strongly  illumined  picture  placed  in  an  in- 
verted position  at  the  shorter  conjugate  focus  of  a single  lens  or  set  of 
lenses,  appears  as  an  erect,  magnified  image  on  a screen  placed  at  the 
longer  conjugate  focus. 

In  the  camera  obscura,  a lens  forms  an  image  of  any  distant  object, 
which  image  is  received  on  a sheet  of  paper  placed  at  its  shorter  con- 
jugate focus.  In  the  compound  microscope  and  in  the  telescope  there 
are  two  lenses  or  sets  of  lenses.  The  one  near  the  object  is  called  the 
object-lens ; the  one  near  the  eye,  the  eye-piece. 


I 


QUESTIONS  FOR  REVIEW. 


247 


la  the  compound  microscope,  the  object  is  placed  at  the  shorter  con- 
jugate focus  of  the  ohject-lens,  and  forms  a magnified  image  near  the 
principal  focus  of  the  eye-lens.  The  eye  sees  the  magnified  image  of 
this  image  through  the  eye-lens. 

In  the  telescope,  the  object  is  placed  before  the  object-lens  at  its 
longer  conjugate  focus,  and  forms  an  inverted  and  diminished  image 
near  the  principal  focus  of  the  eye-lens.  The  eye  views  this  image 
through  the  eye-lens,  by  which  it  is  magnified. 

The  telescope  collects  as  much  more  light  than  the  unassisted  eye 
as  the  area  of  the  object-glass  is  larger  than  the  area  of  the  pupil. 

When  sunlight  is  passed  through  a prism  it  is  separated  into  a great 
number  of  colors  called  a spectrum  ; this  separation  is  called  dispersion. 
There  are  seven  well-marked  colors  in  the  spectrum,  viz. ; violet,  in- 
digo, blue,  green,  yellow,  orange,  and  red.  The  red  is  the  least  and 
the  violet  the  most  refrangible. 

If  all  the  colors  of  the  solar  spectrum  are  again  mixed  together  they 
will  form  white  sunlight. 

The  cause  of  color  is  due  to  certain  of  the  rays  of  sunlight  being  ab- 
sorbed by  the  bodies  on  which  the  light  falls,  and  only  the  rest  of  the 
colors  being  given  off. 

The  rainbow  is  caused  by  the  dispersion  of  sunlight  by  rain-drops. 

Light  and  radiant  heat  differ  from  one  another  only  in  that  the 
vibrations  of  the  ether-waves  that  cause  light  are  more  rapid  than 
those  which  cause  radiant  heat. 


I Questions  for  Review. 

How  is  the  position  of  an  object  affected  by  viewing  it  through  a 
prism  ? What  are  lenses  ? Name  the  three  forms  of  converging  and 
t the  three  forms  of  diverging  lenses. 

I What  is  the  position  of  the  principal  focus,  shorter  conjugate  focus, 
j longer  conjugate  focus,  and  virtual  focus  of  a convex  lens  ? 

Why  do  lenses  cause  the  apparent  size  of  objects  seen  through  them 
to  differ  from  their  real  size  ? 

I What  will  be  the  position  of  the  image  formed  by  a convex  lens, 

! when  the  object  is  placed  between  the  principal  focus  and  the  lens;  at 
j the  shorter  conjugate  focus;  and  at  the  longer  conjugate  focus? 

Describe  the  general  structure  of  the  eye.  Name  the  situation  and 
[ use  of  the  following  parts  of  the  eye,  viz. : the  cornea,  the  aqueous 
i humor,  the  iris,  the  pupil,  the  crystalline  lens,  and  the  vitreous 
\ humor. 


! 


248 


NATURAL  PHILOSOPHY. 


What  is  the  cause  of  near-sightedness  ? What  is  the  cause  of  long- 
sightedness ? How  may  each  be  partially  remedied  ? 

What  are  optical  instruments  ? Describe  the  construction  and  oper- 
ation of  the  following  optical  instruments,  viz. : the  simjile  micro- 
scope, the  photographing  camera,  the  magic  lantern,  and  the  camera 
obscura. 

How  does  the  compound  microscope  differ  from  the  simple  micro- 
scope ? 

What  two  kinds  of  telescopes  are  there?  To  what  does  the  tele- 
scope owe  its  great  penetrating  power  ? How  does  the  refracting 
telescope  differ  from  the  microscope  ? 

How  can  you  prove  that  the  light  of  the  sun  contains  a great  num- 
ber of  different-colored  lights  ? Name  seven  of  the  most  prominent  of 
these  colors.  Define  spectrum ; dispersion. 

Which  of  the  colors  of  the  spectrum  has  the  greatest  refrangibility  ? 
Which  has  the  least  refrangibility?  Which  has  the  greatest  wave- 
length ? Which  has  the  least  wave-length  ? 

What  color  results  from  the  mixing  of  all  the  colors  of  the  solar 
spectrum?  How  may  this  mixing  be  effected? 

Explain  in  full  the  cause  of  color.  What  is  the  cause  of  the  rain- 
bow ? 

What  is  the  only  difference  between  light  and  radiant  heat? 


CHAPTER  III. 

ELECTRICITY.  — ELECTRICAL  CHARGE,  OR 
ELECTRICITY  OF  HIGH  TENSION. 

292.  The  Nature  of  Electricity.  — But  little  is 
kuown  as  to  the  real  nature  of  electricity.  It  is, 
however,  a form  of  energy,  and  all  other  forms  of 
energy  can  readily  be  converted  into  it. 

293.  Varieties  of  Electrical  Energy, — Electricity 
manifests  its  presence  in  a variety  of  ways ; these, 
however,  may  all  be  arranged  under  two  heads : viz., 

1st.  As  a charge. 

2d.  As  a current. 

294.  Electrical  Charge  or  Electricity  of  High 
Tension.  — If  a dry  rod  of  glass  or  stick  of  sealing- 
wax  be  briskly  rubbed  with  a piece  of  silk  or  flannel, 
the  portions  rubbed  will  acquire,  in  addition  to  the 
properties  they  originally  possessed,  the  power  of  at- 
tracting or  repelling  light  objects.  By  means  of  the 
friction,  the  glass  or  wax  has  become  electrified,  that  is, 
has  acquired  an  electrical  charge. 

Bodies  may  acquire  an  electrical  charge  in  a number 
of  ways,  one  of  the  commonest  of  which  is  by  means 
of  friction. 

An  electrical  charge  is  most  manifest  when  it  is 

249 


250 


NATURAL  PHILOSOPHY. 


electricity  of  high  tension,  so  named  from  its  ability 
to  leap  through  distances  in  order  to  overcome  ob- 
stacles placed  in  its  path.  Electricity  of  high  tension 
is  seen  in  its  greatest  development  in  lightning. 

295.  Current  Electricity. — When  a body  contain- 
ing an  electrical  charge  is  brought  into  contact  with 
another  body  through  which  electricity  is  capable  of 
passing,  a current  of  electricity  ensues.  Such  cur- 
rents, however,  are  but  momentary. 

If  a piece  of  copper  and  a piece  of  zinc,  or  two  pieces 
of  any  different  metals  joined  by  a wire,  be  dipped  into  an 
acid  solution  that  acts  chemically  on  one  of  the  metals, 
a continuous  current  of  electricity  will  flow  through  the 
wire  as  long  as  the  metal  is  acted  on  by  the  liquid.  The 
current  flowing  through  the  wire  will  cause  it  to  acquire, 
besides  the  properties  it  originally  possessed,  a number 
of  additional  properties,  among  which  may  be  men- 
tioned its  power  of  attracting  or  repelling  other  wires 
through  which  electrical  currents  are  flowing,  or  of 
attracting  or  repelling  certain  bodies  called  magnets. 

296.  Effects  Produced  by  Electrical  Charge. — -If 

a body  containing  an  electrical  charge,  such,  for  ex- 
ample, as  a rubber  comb,  which  has  been  briskly 
rubbed  with  a silk  handkerchief,  be  brought  near  the 
face,  a creeping  sensation  will  be  experienced,  as 
though  cob-webs  were  touching  it.  If  the  electrified 
body  be  brought  near  a blunt  metallic  body  or  the 
knuckle  of  the  hand,  a faint  bluish  spark  will  pass  to 
the  metal  or  the  hand,  with  a slight  crackling  sound. 

Whenever  any  body  receives  an  electrical  charge 
by  means  of  friction,  both  the  body  rubbed  and  the 
thing  rubbing  it  are  electrified. 

O O 

The  high-tension  electricity  produced  b}''  friction  is 


ELECTRICITY  OF  HIGH  TENSION.  251 


sometimes  caWed.  f rictional  electricity.  Since,  however, 
it  can  be  obtained  in  a variety  of  other  ways,  the  name 
is  inappropriate. 


297.  Conductors  of  Electricity.  — Bodies  differ 
very  considerably  as  to  the  resistance  they  offer  to  the 
passage  of  electricity  through  them.  Some,  like  the 
metals,  are  very  good  conductors.^  while  others,  like 
resins  or  hard  rubber,  are  very  poor  conductors. 

In  the  following  table  a number  of  common  sub- 
stances are  arranged  in  the  order  of  their  conductivity, 
beginning  with  the  best  conductors  and  ending  with 
the  poorest. 


1.  Metals. 

2.  Charcoal. 

3.  Graphite. 

4.  Acids. 

5.  Water. 

6.  Vegetables. 

7.  Animals. 


8.  Linen. 

9.  Cotton. 

10.  Dry  wood. 

11.  Paper. 

12.  Oxides. 

13.  Caoutchouc. 


14.  Dry  paper. 

15.  Silk. 

16.  Glass. 

17.  Wax. 

18.  Eesins. 

19.  Shellac. 


The  conducting  power  of  the  substances  near  the  end 
I of  the  list  is  so  slight  that  they  are  sometimes  called 
non-conductors.  When  a conductor  is  supported  on  a 
I non-conductor  it  is  said  to  be  insulated.^  and  will  then, 
if  the  air  be  dry,  retain  its  excitement  for  a long  time. 

Moist  air  is  a partial  conductor  of  electricity,  while 
dry  air  is  a non-conductor.  All  experiments  in  high- 
tension  electricity  should  therefore  he  tried  in  clear.,  cold, 
dry  weather,  as  in  warm,  damp  weather  the  electrified 
body  rapidly  loses  its  charge. 

298.  Attractions  and  Repulsions  by  Electrified 
Bodies.— The  attractions  and  repulsions  of  light  bod- 
ies by  electrified  bodies  can  be  conveniently  shown  by 


252 


NATURAL  PHILOSOPHY. 


means  of  a pith-ball  suspended  by  a silk  thread  from 
any  suitable  support,  as  shown  in  Fig.  130.  When  an 


Fig.  130.  — Electrical  Attractions  and  Eepnlsions. 


electrified  body,  such,  for  example,  as  a rod  of  glass 
which  has  been  rubbed  with  a dry  silk  handkerchief, 
is  brought  near  the  pith-ball,  the  latter  is  attracted 
to  the  glass,  as  shown  at  A.  As  soon  as  the  pith-ball 
touches  the  glass  rod  it  is  repelled  from  it,  as  shown 
at  A,  and  if  not  allowed  to  touch  the  ground,  or  any 
conducting  body  in  connection  therewith,  will  continue 
to  be  repelled.  If,  however,  it  touches  such  a body,  it 
will  again  be  attracted  to  the  rod,  and  again  repelled. 

If  any  other  electrified  body,  as,  for  example,  a 
piece  of  sealing-wax  rubbed  with  flannel,  be  brought 
near  the  pith-ball  while  it  is  quietly  hanging  from  its 
support,  it  will  be  attracted  and  repelled  the  same  as 
the  glass.  If,  however,  while  the  pith-ball  manifests 
repulsion  for  the  electrified  glass,  the  electrified  seal- 
ing-wax be  brought  near  it,  it  is  at  once  attracted ; or 
if  it  is  repelled  by  the  wax  it  is  attracted  by  the  glass. 

If  any  other  substance  be  electrified  by  friction,  we 
will  find  that  it  acts  either  like  the  glass  or  like  the 
wax ; and  we  therefore  conclude  that  there  are  but  two 
kinds  of  electrical  charge  or  excitement,  viz. ; one  like 


ELECTRICITY  OF  HIGH  TENSION.  253 


tliat  excited  in  glass  rubbed  with  silk,  and  one  like  that 
excited  in  wax  rubbed  with  flannel.  The  former  is 
called  positive  and  the  latter  negative  electricity.  Posi- 
tive electricity  is  generally  represented  by  a -f-;  nega- 
tive by  a — . 

The  law  of  electrical  attractions  and  repulsions  may  be 
stated  as  follows : Bodies  charged  ivith  the  same  hind  of 
electricity  repel  one  another ; those  charged  with  different 
hinds  of  electricity  attract  one  another.  This  law  is  the 
same  as  that  of  magnetic  attractions  and  repulsions. 

299.  Electroscope. — An  electroscope  is  an  instru- 
ment used  to  determine  the 
kind  of  electricity  with 
which  a body  is  charged. 

The  pith-ball  shown  in  Fig. 

130  is  an  electroscope.  A 
better  form,  however,  con- 
sists of  two  small  strips  of 
gold-leaf,  n n,  Fig.  131,  at- 
tached to  a metal  rod  ter- 
minating in  a metal  ball,  c. 

The  gold-leaves  and  rod  are 
placed  in  a glass  jar,  B.,  in- 
side of  which  the  air  can 
be  kept  free  from  moisture.  If  the  ball  be  touched 
by  an  electrified  body,  the  gold-leaves  receive  a charge 
of  the  same  kind  as  that  in  the  electrified  body,  and 
are  therefore  repelled. 

To  determine,  by  means  of  the  electroscope,  the  kind 
of  electricity  with  which  any  bodj^  is  charged,  the  gold- 
leaves  are  repelled  by  a known  kind  of  electricity,  say 
positive,  which  can  readily  be  done  by  holding  a body 
whose  electrical  charge  is  known  to  be  positive,  near 
22 


254 


NATURAL  PHILOSOPHY. 


the  ball,  c.  ISTow,  while  the  leaves  are  thus  diverged, 
bring  the  body,  the  kind  of  whose  charge  is  unknown, 
near  the  ball,  c,  and  watch  the  gold-leaves.  If  they 
divercye  still  farther.,  the  charge  of  the  body  is  posi- 
tive; if,  however,  the  leaves  are  attracted,  its  charge  is 
negative. 


Experiment.  — Cut  out  two  pieces  of  gold  or  silver  paper,  A and 
B,  about  two  inches  square.  Tie  two  pieces  of 
sewing-silk  of  the  same  length  to  holes  at  a i and 
c (f,  as  shown  in  Fig.  132,  and  hang  tliem  to  a rod 
or  other  support,  C,  so  that  they  shall  be  directly 
opposite  each  other,  and  they  will  form  an  excellent 
electroscope.  If  the  leaves  be  touched  by  an  elec- 
trified body,  they  will  be  repelled ; and  if  the  air  be 
dry,  will  continue  to  stand  apart  for  a long  time. 

300.  Positive  and  Negative  Charge, 

— The  kind  of  electricity  produced  by 
rig,  132.— A Sim-  friction,  that  is,  whether  positive  or 
pie  Electroscope,  negative,  depends  on  the  bodies  that 
are  rubbed  together. 

In  the  following  list  of  substances  suitable  for  pro- 
ducing electricity  by  friction,  the  different  substances 
are  so  arranged  that  each  will  be  positively  electrified 
if  rubbed  by  any  body  which it,  but  negatively 
electrified  if  rubbed  by  any  body  which  precedes  it. 


1.  Cat’s  skin. 

2.  Woollea  &brics. 

3.  Glass. 


4.  Cotton. 

5.  Silk. 

6.  The  hand. 


7.  W ood. 

8.  Sealing-wax. 

9.  Hard  rubber. 


Thus,  if  glass  be  rubbed  with  cat’s  skin  or  flannel,  it 
becomes  negatively  electrified ; but  if  rubbed  with  cot-  I 
ton  or  silk  it  becomes  positively  electrified.  | 

The  rubber  is  always  oppositely  electrified  to  the  I 
thing  rubbed.  Glass  rubbed  with  silk  is  positively  1 
electrified,  while  the  silk  is  negatively  electrified.  I 


ELECTRICITY  OF  HIGH  TENSION.  255 

301.  Hypotheses  of  Electricity.  — A number  of 
different  hypotheses  or  suppositions  as  to  the  nature 
of  electricity  have  been  proposed,  none  of  which,  how- 
ever, are  quite  satisfactory ; we  shall  consider  two  of 
the  most  prominent,  viz.:  the  single-fluid  hypothesis.^ 
and  the  double-fluid  hypothesis. 

The  Single- Fluid  Hypothesis  was  first  proposed  by 
Franklin,  an  American  philosopher.  It  ascribes  all 
electrical  phenomena  to  the  presence  of  an  imaginary 
fluid,  the  particles  of  which  are  self-repellent,  but  are 
attracted  by  all  matter,  and  are,  therefore,  supposed  to 
be  present  in  all  bodies.  When  a body  contains  a cer- 
tain amount  of  this  fluid,  we  cannot  detect  the  presence 
of  the  fluid;  but  if  by  any  means  the  quantity  of  the 
fluid  be  either  increased  or  diminished.,  then  the  fluid 
manifests  itself. 

When  a body  has  more  than  its  natural  amount  of 
the  fluid,  it  is  positively  excited  or  charged ; when  it 
has  less  than  its  natural  amount,  it  is  negatively  excited. 
When  glass  is  rubbed  by  silk,  the  glass  is  supposed  to 
take  some  of  the  fluid  from  the  silk,  thus  becoming 
positively,  and  leaving  the  silk,  negatively  electrified. 

The  Double-Fluid  Hypothesis  of  Symmer  and  Du 
Faye,  supposes  the  existence  of  two  different  fluids,  viz., 
the  positive  and  the  negative.  The  particles  of  either 
of  these  fluids  will  repel  particles  of  its  own  kind,  but 
will  attract  particles  of  the  other  kind.  When  both 
fluids  are  present  in  the  same  body,  they  mask  or  neu- 
tralize each  other,  so  that  it  is  only  when  they  are 
separated  that  they  can  produce  an  electrical  charge. 
When  a body  is  electrified  by  any  means,  these  fluids 
are  supposed  to  be  separated  from  each  other.  Thus, 
when  glass  is  rubbed  with  silk,  the  neutral  electricity 
of  each  is  decomposed,  the  glass  gives  its  negative  to 


256 


NATURAL  PUILOSOPHY. 


tlie  silk,  and  tke  silk  its  positive  to  the  glass,  and  tlius 
eack  becomes  electrified. 


It  should  be  borne  in  mind  that  these  are  merely  hypotheses.  We 
do  not  know  whether  electricity  is  a fluid,  or  whether,  like  light  and 
heat,  a wave-motion  of  the  ether  or  of  the  atoms  of  bodies.  The 
hypotheses  are  of  value  on  account  of  the  aid  they  afford  in  connecting 
a number  of  phenomena.  Either  the  single-  or  the  double-fluid  hy- 
pothesis will  explain  nearly  all  the  facts.  For  most  purposes  the  sin- 
gle-fluid hypothesis  is  preferable  to  the  double-fluid  hypothesis. 


7mi 

1 A A 

302.  Induction  of  Electricity. — If  an  insulated  con- 
ductor, as,  for  example,  the  cylinder,  A jB,  Fig.  133,  be 

brought  near  an  elec- 
trified body,  as  the  in- 
^sulated  excited  sphere, 
C,  the  cylinder  will 
become  electrified  by 
induction,andthepith- 
balls  hung  thereon  will 
manifest  repulsion. 

® This  repulsion,  how- 

ever, will  be  unequal,  the  pith-balls  near  the  ends,  A 
and  B,  showing  the  strongest  excitement,  while  those 
at  the  point,  J/,  show  none.  If  C be  charged  with  posi- 
tive electricity,  then  the  end  A will  be  negative  and 
the  end  B positive,  as  may  be  proved  by  means  of  an 
electroscope. 

The  cause  of  induction  can  be  explained  by  either  the 
single-  or  the  double-fluid  hypothesis.  Taking  the  sin- 
gle-fluid hypothesis,  the  free  fluid  in  C disturbs  the  elec- 
trical equilibrium  in  the  cylinder,  and  repels  its  elec- 
tricity to  the  end,  i?,  farthest  from  it.  The  cylinder 
now  has  a deficiency  of  fluid  at  which  end  is  there- 
fore negative,  and  an  excess  at  -B,  which  is  therefore 
positive.  It  contains,  however,  no  more  electricity  than 


ELECTRICITY  OF  HIGH  TENSION.  257 

it  did  at  first,  and  if  C be  now  removed,  the  fluid  will 
flow  from  B towards  C,  and  restore  the  electric  equi- 
librium. If,  however,  anj  part  of  the  cylinder  be 
touched  while  within  the  influence  of  C,  the  repulsion 
of  C will  drive  some  of  the  electricity  from  the  cylin- 
der to  the  ground,  and  when  the  sphere,  C,  is  removed, 
it  will  have  less  than  its  natural  amount  of  fluid  or  will 
be  negatively  charged. 

When  a body,  therefore,  is  permanently  electrified 
hy  induction,  its  electricity  is  of  the  opposite  name  to 
that  of  the  exciting  body.  When  a body  is  electrified 
hy  contact,  it  is  electrified  by  electricity  of  the  same 
name  as  that  of  the  exciting  body. 


303.  Cause  of  the  Attractions  and  Repulsions  of 
Excited  Bodies. — Alternate  attractions  and  repulsions 
of  light  bodies  by  electrified  surfaces  are  the  result  of 
induction.  If  an  un- 
electrified pith-bail, 

B,  be  brought  near 
a conductor.  A,  Fig. 

134,  charged  with 
positive  electricity,  it 
is  at  once  electrified 
by  induction,  and 
the  side  nearer  A, 
being  of  the  opposite 


Fig.  134. 

Cause  of  Eleotrioal  Attraction  and  Eepulsion. 


electricity  to  A,  the  ball  is  attracted  to  the  conductor. 
As  soon  as  it  touches  the  conductor  it  receives  a 
charge  of  free  positive  electricity,  the  same  as  that  of 
the  conductor,  and  is  therefore  repelled  from  it.  If, 
now,  it  should  touch  a body,  G,  in  connection  with  the 
ground,  it  will  again  be  attracted  and  repelled  as  be- 
fore. 

22*  E 


258 


NATURAL  PHILOSOPHY. 


304.  Electrical  Tension.  — All  the  phenomena  of 
electricity  result  from  the  tendency  of  electrified  bodies 
to  regain  their  electrical  equilibrium.  According  to 
the  single-fluid  theory,  the  body  having  an  excess  en- 
deavors to  give  off  some  of  its  fluid  to  the  body  having 
a deficiency.  According  to  the  double-fluid  theory, 
the  positive  fluid  of  one  body  endeavors  to  combine 
with  and  neutralize  the  negative  fluid  of  another  bod3^ 
The  condition  of  strain  produced  by  these  tendencies 
is  called  the  electrical  tension. 

305.  Distribution  of  an  Electrical  Charge. — When 
an  insulated  conductor  is  electrified,  the  electricity  is 
all  apparently  on  the  outside  of  the  body.  An  insulated 
hollow  sphere,  provided  with  a hole  in  the  top,  is 
charged  with  electricity.  To  determine  the  distribu- 
tion of  the  charge,  a proof  plane which  consists  of  a 
small  metallic  disc  attached  to  the  end  of  a glass  rod, 
may  touch  the  outside  of  the  sphere  at  any  point,  and 
will  always  take  away  a small  charge  of  electrieity,  as 
may  be  proved  by  an  electroscope.  But  if  touched  to 
any  part  of  the  inside  of  the  sphere,  care  being  taken 
not  to  allow  it  to  touch  the  edge  of  the  hole  in  insert- 
ing or  removing  the  proof  plane,  it  will  be  found  to 
contain  no  electricity,  all  the  charge  of  the  sphere  resid- 
ing on  its  surface. 

It  must  be  remembered,  however,  that  this  is  only 
true  for  an  electrical  charge.  When  the  electricity  is 
in  motion,  or  is  an  electrical  current.,  then  it  passes 
through  the  whole  substance  of  the  conductor.  A 
hollow  wire  will  not  conduct  electricity  any  better 
than  a solid  wire  of  the  same  weight  and  length. 

306.  The  Influence  of  Points.  — In  an  insulated 
excited  sphere  the  depth  of  the  electrical  fluid,  as 


ELECTRICITY  OF  HIGH  TENSION.  259 


shown  by  the  tension,  is  the  same  over  all  parts  of  the 
surface ; but  if  the  excited  conductor  be  egg-shaped, 
then  the  tension  is  greatest  near  the  point  of  the  egg, 
and  this  will  be  found  to  increase  the  sharper  the  point, 
so  that  when  the  end  is  very  pointed,  the  tension  may 
become  sufficiently  powerful  to  enable  the  electricity 
to  escape  into  the  air.  Conductors  intended  to  retain 
an  electrical  charge  are,  therefore  rounded  so  as  to 
avoid  the  presence  of  points. 


Fig.  135. 

The  Electrophorus  (charging). 


307.  Electrical  Machines.  The  Electrophorus. — 

The  simplest  form  of  electrical  machine  is  the  electro- 
phorus. It  consists  of  a plate, 

J.,  of  brass  or  some  other  metal, 
attached  to  a glass  rod,  and  a 
plate  of  resin,  i?,  placed  in  a me- 
tallic dish.  The  resin  is  excited 
with  negative  electricity  by  rub- 
bing it  briskly  with  a piece  of 
cat-skin.  The  plate,  J,  is  then 
held  by  the  glass  handle,  and 
placed  over  the  resin,  as 
shown  in  Fig.  135,  and  be- 
comes electrified  by  induc- 
tion, the  side  nearest  the 
resin  being  positively,  and 
the  side  farthest  from  it  neg- 
atively, charged.  If  A is  now 
raised  from  the  resin  without 
previously  touching  it  to  any 
conductor,  it  will  be  found  to 
possess  no  charge,  since  the 
electricity  on  the  positive  side  of  the  disc  would  flow 
towards  the  negative  side,  and  neutralize  it.  If,  how- 


Fig.  136. 

The  Electrophorus  (discharging). 


260 


NATURAL  rillLOSOPHT. 


ever,  the  disc  be  touched  hy  the  finger,  it  will,  when 
raised  from  the  resin,  contain  free  positive  electricity. 

Experiment. — An  excellent  electrophorus  may  be  made  as  follows: 
Place  in  a tin  pie-plate  equal  parts  of  rosin  and  gum-shellac,  sufficient 
in  quantity  to  nearly  fill  the  plate  when  melted ; place  the  dish  over  a 
fire,  and  very  gradually  melt  the  rosin  and  shellac,  at  the  same  time 
stirring  with  a stick  to  break  air  bubbles.  When  melted,  set  the  dish  on 
a flat  support  to  cool.  Now  cut  a disc  of  wood  smaller  in  diameter  than 
the  rosin-plate;  bore  a hole  in  the  middle  of  the  wood,  and  cement  in  it  a 
glass  rod  or  tube.  Paste  the  tin-foil  over  the  wooden  disc,  covering  it 
completely,  and  remove  any  rough  ends  by  smoothing  the  foil  with  the 
finger-nail.  Now  rub  the  rosin  plate  with  a bit  of  silk  or  cat-skin,  and 
place  the  tin-foil  disc  on  the  rosin,  and  touch  it  with  the  hand.  On 
removing  the  disc  a spark  of  positive  electricity  may  be  taken  from  it 
by  the  knuckle.  When  the  rosin-plate  is  once  charged  by  rubbing, 
an  indefinite  number  of  sparks  may  be  obtained  by  placing  the  disc 
each  time  on  the  rosin,  and  touching  with  the  finger,  as  before.  When 
the  air  is  dry  and  cold,  as  in  winter,  and  the  glass  handle  clean  and 
dry,  sparks  of  considerable  length  can  be  obtained.  | 

Caution.  — Do  not  overheat  the  mixtures  of  rosin  and  shellac,  or  j 
bubbles  will  form  and  spoil  the  surface.  Before  using  the  electro-  ! 
phorus,  be  sure  that  everything  is  dry.  I 

A very  great  number  of  electrical  macliines  have  been  I 
devised.  We  will  describe,  however,  only  one,  viz. : i 

308.  The  Plate  Electrical  Machine. — This  machine  i 

consists  of  a circular  plate  of  glass.  A,  Fig.  137,  mounted  | 
on  an  axis,  B,  on  which  it  can  be  turned  by  means  of  | 
an  insulated  handle,  C.  At  D,  a rubber  made  of  piano  i 
felting  or  chamois  skin  is  pressed  by  brass  springs  firmly  i 
against  the  plate.  The  rubber  has  generally  a mixture  I 
of  tin  and  mercury,  called  an  amahjam,  spread  over  its  ; 
surface,  and  is  in  electrical  contact  with  an  insulated  j 
conductor,  A,  called  the  negative  conductor.  A series  i 
of  metallic  points.  A,  connected  with  an  insulated  con- 
ductor, 0,  called  the  positive  or  prime  conductor,  is  I 
placed  near  that  part  of  the  plate  diametrically  opposite  | 
the  rubber,  as  shown.  On  turning  the  handle,  the  fric-  | 


ELECTRICITY  OF  HIGH  TENSION.  261 


tion  of  tlie  rubber  on  tbe  glass  causes  the  glass  to  become 
positive  and  tlie  rubber  negative.  The  negative  con- 
ductor is  now  charged  by  the  rubber,  while  the  electric- 
ity on  the  glass,  coming  opposite  the  points,  charges  the 


Fig.  137.  — Plate  Electrical  Machine. 

conductor  connected  with  them  positively.  The  lower 
half  of  the  plate  is  loosely  covered  by  a bag  of  silk,  S. 
When  only  positive  electricity  is  desired,  a chain,  IF, 
connects  the  negative  conductor  with  the  ground. 


' 309.  The  Condensation  of  Electricity. — When  an 

insulated  con- 
ductor, Fig. 

' 138,  is  brought 

into  contact 
with  the  con- 
ductor of  an 
electrical  ma- 
chine, it  can- 
not receive  a 
charge  hav- 


ing a tension  Fig.  138.  — The  Condenser  of  ilpinns. 

higher  than  that  of  the  machine.  If,  however,  it  be 


262 


NATURAL  PHILOSOPHY. 


placed  near  a second  insulated  conductor,  B,  separated 
from  it  by  a plate  of  glass,  (7,  and  tbe  two  conductors 
moved  towards  each  other  until  they  touch  the  glass 
plate,  and  the  plate,  i?,  be  connected  with  the  earth, 
then  A can  receive  a charge  whose  tension  is  greater 
than  that  of  the  electrical  machine.  This  apparatus, 
shown  in  Fig.  138,  is  called  the  condenser^  and  the 
process  by  which  A and  B receive  this  high-tension 
charge,  the  condensation  of  electricity. 

The  manner  in  which  the  charge  is  given  to  A and 
B is  as  follows : Suppose  A positive ; then  by  induc- 
tion B becomes  negative,  and  its  free  positive  electricity 
is  repelled  to  the  ground.  The  positive  and  negative  | 
electricities  in  A and  B neutralize  each  other’s  effects,  | 
thus  leaving  the  plates  free  to  receive  a new  charge,  i 
A can  therefore  receive  more  electricity  from  the  ! 
machine,  which  again  acts  by  induction  through  C,  | 
and  allows  more  electricity  from  A and  i?  to  be  i 
neutralized.  In  this  way  the  tension  of  the  charge  : 
in  the  condenser  may  become  very  great,  and  in-  ! 
deed  often  becomes  sufficiently  intense  to  pierce  the  i 
glass.  \ 

To  discharge  the  condenser.^  it  is  only  necessary  to  j 
join  the  plates  A and  B by  any  conductor,  when  i 
the  attractions  of  the  opposite  electricities  cause  them 
to  flow  through  the  conductor  and  neutralize  each  j 
other. 

310.  The  Leyden  Jar. — The  condenser  is  generally  i 
made  in  the  form  of  the  Leyden  jar,  which  is  shown 
in  Fig.  139,  where  the  lower  part  of  a glass  jar  is 
coated  on  the  inside  and  outside  with  tin-foil.  The 
inside  coating  is  in  connection  with  the  brass  knob.  A, 
by  means  of  the  chain,  E.  To  charge  the  jar  it  is  held 


ELECTRICITY  OF  HIGH  TENSION.  263 


in  tlie  hand,  which  grasps  the  outer  coating  while  the 
knob,  4,  is  brought  near  the  conductor 
of  an  electrical  machine,  and  a number 
of  sparks  passed  into  the  jar.  If  now 
a person  place  one  hand  on  the  outer 
coating,  and  the  other  on  the  knob,  J., 
the  opposite  electricities  pass  through 
his  body  and  give  a more  or  less  severe 
shock. 

A battery  of  Leyden  jars  consists  of  a 
number  of  jars  having  their  inner  and  outer  coatings 
respectively  connected  with  each  other.  The  shock 
from  a Leyden  jar  may  be  passed  through  a number 
of  persons  joined  hand  to  hand,  the  person  at  one  end 
of  the  line  touching  the  outside  coating  of  the  jar,  and 
the  person  at  the  other  end  the  knob.  The  shock  from 
a very  large  battery  may  prove  fatal. 


Fig.  139. 

The  Leyden  Jar. 


311.  Effects  of  the  Electric  Discharge.  — The  ef- 
fects produced  by  the  electric  discharge  are  physio- 
logical.^ luminous.,  thermal,  mechanical,  and  chemical. 

The  physiological  effects  are  partly  seen  in  the  shock 
produced  by  the  passage  of  the  discharge  through  the 
body. 

The  luminous  effects  are  seen  in  the  bright  spark  that 
always  accompanies  the  discharge.  The  spark  is  more 
brilliant  the  greater  the  tension  of  the  electricity.  Its 
color  depends  on  the  kind  of  conductor  between  which 
the  spark  passes,  and  on  the  kind  and  density  of  the 
gas  through  which  it  passes. 

The  thermal  or  heating  effects  are  seen  in  the  heat 
produced  whenever  the  passage  of  the  discharge  is  re- 
sisted as  by  passing  it  through  a very  thin  wire  or 
gold-leaf,  which  may  be  volatilized  by  the  heat.  The 


264 


NATURAL  PHILOSOPHY. 


fact  tbat  the  spark  gives  off  light  proves  that  it  is 
itself  hot.  A spark  from  ah  electrophorus  will  ignite 
a gas  flame. 

The  mechanical  effects  are  seen  in  the  violent  frac- 
tures or  tearing  produced  when  a powerful  discharge 
is  sent  through  any  poor  conducting  substance.  Thick 
glass  plates  may  be  pierced,  and  boards  fractured  by 
the  discharge  from  a powerful  Leyden  battery. 

The  chemical  effects  are  seen  in  the  combinations  or 
decompositions  produced  by  the  discharge.  A small 
quantity  of  the  nitrogen  and  oxygen  in  the  air  com- 
bine and  form  nitric  acid,  on  passing  a number  of  dis- 
charges through  moist  air.  A spark  of  electricity  sent 
through  a piece  of  paper  moistened  with  iodide  of  po- 
tassium stains  the  paper  brown  where  the  spark  pierces 
it,  by  a deposit  of  free  iodine. 

312.  Atmospheric  Electricity.  — The  atmosphere 
almost  always  contains  a charge  of  positive  electricity, 
though  it  sometimes  changes  rapidly  to  negative  on 
the  approach  of  clouds  or  in  stormy  weather.  Its  in- 
tensity is  least  near  the  earth’s  surface,  and  increases 
with  the  altitude. 

313.  Lightning.  — The  moisture  in  clouds  enable 
them  to  collect  the  free  electricity  of  the  air  ; for,  since 
damp  air  is  a conductor,  the  clouds  collect  the  elec- 
tricity of  the  air  through  which  they  are  moving,  and 
allow  the  electricity  to  pass  through  and  collect  on 
their  surfaces,  until  considerable  tension  is  acquired. 
When  such  clouds  come  near  the  earth,  the  ground 
below  them  becomes  oppositely  electrified  by  induc- 
tion ; and  when  these  opposite  charges  acquire  suffi- 
cient tension,  they  discharge  into  each  other  through 
the  air  : the  flash  which  accompanies  the  discharge  is 


ELECTRICITY  OF  HIGH  TENSION.  265 


called  lightning.  An  electrified  cloud  sometimes  dis- 
charges into  another  cloud  which  is  oppositely  elec- 
trified. 

The  thunder  which  follows  the  lightning  is  caused 
by  the  violent  disturbance  in  the  air  produced  by  the 
electricity  moving  rapidly  through  it,  or  by  the  rapid 
formation  and  condensation  of  vapor  on  the  passage 
of  electricity  through  drops  of  rain. 

314.  Lightning-Rods.  — Lightning-rods,  first  pro- 
posed by  Franklin,  consist  of  stout  rods  of  iron,  or 
preferably  of  copper,  attached  to  the  outside  of  the 
building  to  be  protected,  and  extending  some  little 
distance  above  its  highest  point.  The  upper  end  of 
the  rod  should  be  pointed,  and  its  lower  end  should 
extend  deep  into  the  ground.^  until  it  meets  permanently 
damp  earth,  or  some  conductor  of  electricity.  If  under- 
ground water-  or  gas-pipes  are  in  the  neighborhood,  it 
is  well  to  connect  the  rod  to  them.  If  the  roof  of  the 
building  be  of  metal,  such  as  tin  or  copper,  it  should  he 
connected  with  the  rod.  The  rod  should  be  of  sufficient 
thickness  to  conduct  to  the  earth,  without  being  melted, 
the  heaviest  discharge  that  may  strike  the  building.  A 
solid  rod  is  to  he  preferred.^  as  an  electrical  current  passes 
through  the  whole  mass  of  the  conductor.^  and  not  only 
over  the  surface. 

A lightning-rod  does  not  always  protect  the  build- 
ing by  conducting  the  discharge  from  the  cloud  to  the 
earth.  More  generally  it  acts  by  quietly  discharging 
the  cloud ; becoming,  oppositely  electrified  by  the 
cloud,  it  then  quietly  discharges  this  opposite  elec- 
tricity into  the  cloud,  thus  neutralizing  its  charge. 
A lightmng-rod  not  well  electrically  connected  with  the 
earth,  is  more  a source  of  danger  than  of  protection, 
23 


266 


NATURAL  PHILOSOPHY. 


since  it  attracts  the  discharge  without  being  able  to  safely 
conduct  it  to  the  earth. 


315.  Experiments  in  Frictional  Electricity. — Quite 
a number  of  entertaining  and  instructive  electrical 
experiments  may  be  shown  with  easily  contrived  ap- 
paratus. A few  of  these  will  be  mentioned.  The 
student  is  earnestly  advised,  as  far  as  possible,  to  make 
the  information  his  own,  by  trjdng  and  verifying  the 
facts  by  experiment. 


Experiment — Cut  a piece  of  elder  pith  into  a ball,  tie  it  to  a piece 
of  silk,  and  suspend  it  from  any  support.  Try  the  effect  of  rubbing 
different  bodies  together,  and  hold  them  near  the  pith-ball,  to  see 
whether  they  are  electrified. 

Experiment.  — Use  the  gilt  paper  electroscope,  described  in  a pre- 
vious experiment,  and  see  what  kind  of  electricity  you  have  produced. 
As  far  as  you  can,  test  the  correctness  of  the  table  given  in  par.  300. 

Experiment.  — Cut  two  discs  of  gilt  paper,  a and  h,  as  large  as  silver 
quarter  dollars,  and  connect  them,  as  shown  in  the  Ex.  on  page  254, 
but  with  linen  or  cotton  thread.  Stick  a wire  through  a cork  of  a 
wide-mouthed  bottle,  and  tie  the  threads  to  the  end  of  the  wire,  leav- 
ing the  discs  hanging  by  about  inch  of  thread. 
Solder  a smooth  metal  button.  A,  to  the  end  of  the 
wire,  and  put  the  cork  in  the  bottle,  so  that  the 
pieces  of  paper  shall  be  inside  the  bottle,  as  shown  in 
Fig.  140,  first,  however,  being  sure  that  the  bottle  is 
perfectly  dry  by  heating  it  on  a warm  stove.  The 
cork  also  must  be  perfectly  dry.  Run  melted  sealing- 
wax  over  the  top  of  the  cork,  so  as  to  prevent  any 
moist  air  from  afterwards  getting  into  the  bottle. 
This  apparatus  will  now  serve  as  an  electroscope. 

Experiment. — Attach  a long  metallic  wire  to  the  button,  A,  of  the 
electroscope  just  made.  Excite  the  electrophorus  described  on  p.  260, 
and  touch  the  far  end  of  the  wire  with  the  tin-foil  disc.  The  leaves, 
a and  b,  will  at  once  diverge,  showing  that  the  wire  has  conducted 
the  electricity  to  them.  Try  the  same  with  a dry  silk  thread,  and 
prove  that  it  is  a poor  conductor. 

Experiment.  — In  the  corners  of  a square  piece  of  wood  bore  four 
holes,  large  enough  to  insert  the  necks  of  four  porter  or  ale  bottles. 
Stand  a person  on  this  insulated  stool,  and  charge  him  with  electricity 


Pig.  140. 

An  Electroscope. 


ELECTRICITY  OF  HIGH  TENSION.  267 


from  the  electrophorus  by  giving  him  15  or  20  sparks  from  the  tin-foil 
disc.  If,  now,  the  knuckle  be  approached  to  any  part  of  his  body,  an 
electrical  spark  will  pass  from  it  to  the  knnckle. 

Experiment.  — Place  a person  on  the  stool  and  charge  him  as  be- 
fore. He  can  now  light  the  gas  with  the  spark  from  his  finger. 

Experiment.  — Place  a person  on  the  stool,  and  let  him  hold  the 
electroscope.  Fig.  140,  in  his  hand,  with  his  finger  on  the  knob,  A. 
Then  strike  him  on  the  hack  with  a piece  of  cat-skin ; at  every  stroke 
the  leaves,  a and  6,  will  be  seen  to  diverge. 

Experiment. — Suspend  by  silk  threads,  a and  6,  as  shown  in  Fig.  141, 
a tomato-can,  free  from  sharp  edges  ; attach  at  the  lower 
end,  by  linen  or  cotton  thread,  pith- halls,  c and  d,  and 
you  have  an  insulated  conductor. 

Experiment.  — Make  another  snch  conductor,  and 
try  the  effects  of  induction  as  described  in  par.  302. 

Experiment.  — Excite  the  electrophorus,  p.  260,  and 
convince  yourself  that  it  is  working  and  giving  sparks. 

Get  some  one  to  hold  the  point  of  a pin  near  it  when 
you  are  trying  to  get  a spark  from  it,  and  the  pin 
will  discharge  the  tin-foil  disc,  and  no  spark  will  pass  from  it  to  the 
hand. 

Experiment. — To  make  a Leyden  jar,  select  a candy  jar  with  a wide 
mouth,  preferably  with  glass  of  a greenish  hue,  and  paste  tin-foil  on 
the  inside  and  outside,  as  explained  in  par.  310.  Insert  a dry  cork  in 
the  mouth,  and  run  a wire  through  the  cork  down  into  the  jar,  until  it 
touches  the  inner  coating.  Attach  a smooth  metal  button  to  the  top 
of  the  wire.  Charge  the  jar  from  the  electrophorus,  and  take  a shock. 

Experiment. — Get  a smooth  pine  board,  larger  than  a sheet  of  letter 
paper.  Heat  the  board  and  sheet  of  paper  before  a fire.  Then  place 
the  paper  on  the  board,  and  stroke  it  briskly  with  a piece  of  India- 
rubber,  such  as  is  used  for  erasing  lead-pencil  marks.  The  paper  will 
become  strongly  electrified.  Now  lift  it  from  the  board  by  one  of  its 
edges,  and  bring  it  near  the  wall,  and  it  will  be  at  once  attracted  to 
the  wall,  and  will  cling  to  it  for  several  minutes. 

Experiment.  — Electrify  the  paper  as  before,  and  while  it  is  on  the 
board,  cut  it  into  strips.  Take  hold  of  all  the  strips  at  one  end,  and 
lift  them  from  the  hoard.  Their  lower  ends  will  be  repelled,  and  will 
stand  out  from  one  another  in  a very  amusing  manner. 

Experiment. — Electrify  a piece  of  paper  as  before.  Eemove  it  from 
the  hoard,  and  lay  a pith-ball  on  it.  The  ball  will  either  be  thrown 
off  the  paper  at  once,  or  will  run  to  the  lower  side  of  the  paper,  and 
will  then  be  shot  off  from  it. 


Fig.  141. 
An  Insulated 
Conductor. 


268 


NATURAL  PHILOSOPHY. 


Experiment.  — Place  a metal  waiter  on  top  a dry  glass  goblet. 
Electrify  the  paper  and  place  it  on  the  waiter.  Apply  the  knuckle 
to  the  edge  of  the  waiter,  and  a spark  will  pass  to  it.  Eemove  the 
paper  by  the  edge,  and  another  spark  can  be  taken  from  the  paper. 

Experiment. — Support  a clean  dry  pane  of  window-glass,  one  inch 
or  so  above  the  surface  of  a table,  by  resting  the  edges  on  sticks  of  wood. 
Place  three  or  four  pith-balls  under  the  glass,  and  rub  the  top  of  the 
glass  briskly  with  a piece  of  silk  or  cat-skin.  The  balls  will  move 
about  in  a curious  manner,  and  some  will  probably  stick  to  the  glass. 
Now  stop  rubbing,  and  hold  the  finger  near  the  glass  above  the  balks, 
and  they  will  at  qnce  fall.  Since  the  glass  is  not  a conductor  of  elec- 
tricity, and  only  the  top  is  rubbed,  the  pith-balls  must  be  electrified 
by  induction. 

Syllabus. 

Electricity  manifests  its  presence  either  as  a charge  or  as  a current. 

A body  that  possesses  an  electrical  charge  is  said  to  be  electrified. 
An  electrified  body  acquires  the  property  of  attracting  and  repelling 
light  bodies. 

A body  may  be  electrified  in  a number  of  different  ways,  one  of  the 
simplest  of  which  is  by  friction. 

An  electrical  charge,  like  that  produced  by  friction,  possesses  the 
ability  of  leaping  through  short  distances,  in  order  to  overcome  ob- 
stacles placed  in  its  path  ; it  is,  therefore,  commonly  called  electricity 
of  high  tension. 

Good  electrical  conductors  are  such  as  readily  permit  the  passage 
through  them  of  an  electrical  current.  Substances  that  will  not  readily 
permit  the  passage  through  them  of  such  a current  are  called  non-con- 
ductors. 

A body  is  said  to  be  insulated  when  it  is  supported  by  a poor  con- 
ductor. 

There  are  two  kinds  of  electrical  charge,  viz.,  positive  and  negative. 
Electricities  of  the  same  kind  repel  each  other ; those  of  opposite 
kinds  attract  each  other.  The  kind  of  electricity  with  which  a body 
is  electrified  is  determined  by  means  of  the  electroscope. 

Glass  rubbed  with  cat-skin  becomes  negatively  electrified;  but  when 
rubbed  with  silk,  positively  electrified. 

A body  which  opposes  or  offers  but  little  resistance  to  the  passage 
of  electricity  through  it  is  called  a conductor.  Substances  differ 
greatly  in  their  conducting  power  for  electricity  ; some  are  very  good 
conductors,  while  others  are  very  poor  conductors. 


QUESTIONS  FOR  REVIEW. 


269 


According  to  the  single-fluid  electrical  hypothesis,  a ‘body  is  posi- 
tively excited  when  it  has  more  than  its  natural  quantity  of  electrical 
fluid,  and  negatively  excited  when  it  has  less  than  its  natural  quantity. 

According  to  the  double-fluid  hypothesis,  there  are  two  kinds  of 
electrical  fluids,  — positive  and  negative, — which  are  generally  com- 
bined with  each  other.  Electrical  excitement  is  produced  by  sepa- 
rating the  two  fluids  from  one  another. 

When  an  insulated  conductor  is  approached  to  a positively  excited 
conductor,  it  becomes  electrified  by  induction.  Its  end  nearest  the 
excited  conductor  becomes  negative,  and  the  end  farthest  from  it  posi- 
tively excited.  If  it  be  now  touched,  it  loses  positive  electricity,  and 
becomes  permanently  excited  negatively. 

The  attractions  and  repulsions  of  light  bodies  by  electrified  surfaces 
are  caused  by  induction. 

An  electrical  charge  resides  only  on  the  surface  of  a conductor ; an 
electrical  current  passes  through  the  entire  mass  of  the  body. 

A point  attached  to  an  electrified  conductor  acquires  such  a high 
electric  tension  that  it  will  quietly  discharge  the  conductor. 

In  the  electrophorus,  the  electricity  is  produced  in  the  resin  by  fric- 
tion ; but  in  the  metallic  disc  by  induction. 

Condensers  of  electricity  enable  the  electricity  collected  in  a conduc- 
tor to  acquire  a higher  tension  than  that  of  the  electrical  machine  by 
which  they  are  charged ; they  operate  by  induction.  The  condenser 
is  generally  made  in  the  form  of  the  Leyden  jar. 

The  effects  of  the  electric  discharge  are,  1st.  Physiological ; 2d.  Lu- 
minous ; 3d.  Thermal ; 4th.  Mechanical ; and,  5th.  Chemical. 

The  atmosphere  always  contains  free  electricity,  which  is  generally 
positive.  Lightning  is  caused  by  the  discharge  of  a cloud  to  the  ground, 
or  to  a neighboring  cloud.  The  thunder  is  caused  by  violent  disturb- 
ance in  the  air,  produced  by  the  lightning  passing  through  it. 

Lightning-rods  protect  the  buildings  on  which  they  are  placed,  either 
by  conducting  the  discharge  to  the  earth,  or  by  quietly  neutralizing 
the  electrified  cloud  by  discharging  opposite  electricity  into  it. 

Questions  for  Review. 

Name  the  principal  varieties  of  electrical  energy. 

Define  electrical  charge.  When  is  a body  said  to  be  electrified  ? 
Name  some  of  the  effects  produced  by  an  electrified  body. 

What  is  meant  by  current  electricity?  Name  any  source  of  current 
electricity. 

23* 


270 


NATURAL  PniLOSOPHY. 


Distinguish  between  conductors  and  non-conductors  of  electricity. 

Name  some  good  conductors.  Name  some  poor  conductors.  When 
is  a conductor  said  to  be  insulated  ? 

Describe  the  attractions  and  repulsions  produced  by  an  electrified 
body.  State  the  law  of  electrical  attractions  and  repulsions. 

Describe  the  electroscope.  For  what  is  it  used?  Describe  the  con- 
struction of  a simple  electroscope. 

What  kind  of  electricity  will  be  produced  in  glass  by  rubbing  it  with 
silk?  With  cat-skin ? With  cotton? 

Describe  in  full  the  single-fluid  electrical  hypothesis;  the  double- 
fluid electrical  hypothesis. 

Explain  what  is  meant  by  induction  of  electricity.  Has  a body 
when  electrified  by  induction  any  more  electricity  than  it  originally 
contained?  How  may  a body  be  permanently  electrified  by  induction? 

Define  electrical  tension.  Prove  that  an  electrical  charge  resides 
only  on  the  surface  of  a conductor.  Is  this  true  also  of  an  electrical 
current? 

Explain  in  full  the  cause  of  the  attractions  and  repulsions  of  light 
bodies  by  an  electrified  surface. 

What  effects  have  points  on  electrical  tension  ? 

Describe  the  construction  and  operation  of  the  electrophorus  and  of 
the  plate  electrical  machine. 

Explain  fully  the  construction  and  operation  of  the  condenser. 
What  is  meant  by  the  condensation  of  electricity  ? 

Describe  the  Leyden  jar.  How  is  the  Leyden  jar  charged?  How 
is  it  discharged  ? 

Enumerate  the  effects  produced  by  the  electric  discharge. 

What  is  meant  by  a Leyden  battery  ? 

How  does  the  electrical  charge  accumulate  in  a cloud  ? 

By  what  is  lightning  caused?  What  causes  thunder? 

Describe  the  construction  of  a lightning-rod.  How  do  lightning- 
rods  protect  the  buildings  on  w'hich  they  are  placed  ? What  precau- 
tions are  necessary  in  order  to  obtain  an  efficient  lightning-rod  ? 


CHAPTER  IV. 

CURRENT  ELECTRICITY. 

316.  Sources  of  Current  Electricity.  — There  are 
various  sources  of  electrical  current,  the  principal  of 
which,  however,  may  he  arranged  under  three  heads, 
viz. : 

1st.  Currents  produced  by  chemical  action,  or  voltaic 
currents. 

2d.  Currents  produced  by  the  action  of  heat,  or 
thermo-electric  currents ; and 

3d.  Currents  produced  by  the  motion  of  magnets, 
or  magneto-electric  currents. 

We  will  consider  in  this  chapter  the  first  two  of 
these  sources. 

317.  Voltaic  Currents. — We  have  already  seen  that 
if  a piece  of  zinc  and  copper  joined  by  a wire  be  dipped 
into  any  liquid  which  will  act  chemically  on  either 
metal,  a current  of  electricity  will  be  produced.  An 
electrical  current  will  be  produced  when  any  two 
dissimilar  metals  are  used  in  this  manner,  but  it  will 
be  found  by  trial  that  certain  metals  used  in  connec- 
tion with  certain  acid  liquids  will  produce  the  greatest 
current. 

Any  two  metals  that  are  used  together  for  this  pur- 

271 


272 


NATURAL  PHILOSOPHY. 


pose  form  wliat  is  called  a voltaic  couple.^  and  the  entire 
arrangement  a voltaic  cell. 

All  chemical  action  is  attended  with  a disturbance  of  electrical 
equilibrium.  Of  different  kinds  of  chemical  action,  that  which  occurs 
between  metals  and  acid  liquids  is  found  to  be  the  best  suited  for  the 
development  of  electrical  current. 

318.  Galvani  and  Volta. — The  production  of  elec- 
tricity by  chemical  action  was  first  noticed  by  Galvani, 
an  Italian  physiologist,  who  erroneously  ascribed  the 
effects  it  caused  to  a vital  fluid.  He  was  making 
experiments,  in  which  he  used  the  legs  of  recently 
killed  frogs.  Hanging  them  against  an  iron  balustrade, 
he  noticed  that  whenever  the  metal  touched  a large 
nerve  in  the  frog,  and  so  brought  it  into  contact  with 
the  muscles  of  the  leg,  that  the  legs  were  violently 
twitched,  as  though  in  pain.  He  thought  that  these 
movements  were  caused  by  a vital  fluid  which  came 
out  of  the  nerve,  and  flowed  through  the  iron  to  the 
muscles. 

Volta,  a distinguished  physicist,  showed  that  these 
movements  of  the  frogs’  legs  were  due  to  electricity, 
and  constructed  an  arrangement,  called  a pile  or  bat- 
tery, by  means  of  which  electricity  could  be  readily 
produced.  Voltaic  electricity  was  so  named  after  its 
discoverer.  It  is  sometimes  called  galvanic  electricity, 
though  not  so  properly,  since  this  might  imply  a belief 
in  Galvani’s  idea  of  its  being  the  vital  fluid. 

319.  The  Simple  Voltaic  Cell. — A simple  voltaic 
cell  consists  of  two  plates  of  different  metals  immersed 
in  a liquid  which  can  readily  act  on  one  of  them,  and 
connected  outside  of  the  liquid  by  a wire  of  some  good 
conducting  substance. 

One  of  the  simplest  forms  given  to  the  voltaic  cell 
is  seen  in  Fig.  1-12,  where  a plate  of  zinc  and  a plate 


CURRENT  ELECTRICITY. 


273 


of  copper  are  immersed  in  water,  rendered  sour  by 
sulpliuric  acid,  and  connected  outside  the  liquid  by 
means  of  a copper  wire,  M. 

If  the  zinc  be  pure,  no  action  be- 
tween the  liquid  and  either  metal 
occurs  as  long  as  the  metals  do  not 
touch  eo.ch  other.  If,  however,  they 
■ are  made  to  touch  each  other,  either 
in  or  out  of  the  liquid,  then  an  action 
takes  place  between  the  liquid  and 
the  zinc,  and  hydrogen  gas,  produced 
by  the  decomposition  of  the  water,  is  A Simple  Voltaic  Cell, 
seen  to  escape  in  minute  bubbles  from  the  copper,  and 
a current  of  electricity  continues  to  flow  from  the  zinc 
to  the  copper  as  long  as  the  chemical  action  continues. 

The  contact  of  the  plates  outside  of  the  liquid  may  be 
made  either  directly,  by  allowing  them  to  rest  against 
one  another,  or  by  means  of  a wire  of  some  good  con- 
ducting substance,  as  copper.  This  wire  may  be  many 
miles  long,  but  as  soon  as  the  ends  are  brought  to- 
gether, the  liquid  acts  on  the  zinc,  bubbles  escape  from 
the  copper,  and  an  electrical  current  is  produced. 

320.  The  Voltaic  Circuit.  — The  direction  of  the 
current  of  the  simple  battery  cell  just  described,  is 
from  the  zinc  through  the  liquid  to  the  copper,  and 
through  the  conducting  wire  outside  of  the  liquid 
back  again  to  the  zinc,  thus  completing  a circuit,  called 
the  voltaic  circuit. 

We  make  or  complete  the  circuit  when  we  connect 
the  zinc  and  copper  by  means  of  the  conducting  wire, 
and  we  break  the  circuit  when  we  disconnect  them  by 
breaking  or  separating  the  conducting  wire  at  any 
point.  No  current  flows  when  the  circuit  is  broken. 

S 


274 


NATURAL  PHILOSOPHY. 


The  current  immediately  begins  to  flow  when  the  circuit 
is  completed.  No  matter  liow  long  the  connecting  wire 
may  be,  if  the  circuit  be  broken  at  any  point  of  its 
length  the  current  at  once  ceases. 

321.  The  Polarity  of  the  Battery  — Electrodes. — 

In  any  voltaic  cell  the  current  always  flows  through 
the  liquid  from  the  metal  most  acted  on  to  the  metal 
least  acted  on.^  and  out  of  the  liquid  in  the  opposite 
direction. 

In  a voltaic  cell,  in  which  zinc  and  copper  are  em- 
ployed, the  zinc  is  positive  in  the  liquid  and  negative 
outside  of  it,  and  the  copper  is  negative  in  the  liquid  and 
positive  outside  of  it.  If  the  wire  connecting  the  plates 
be  broken  at  any  place,  positive  electricity  accumulates 
at  one  of  the  ends,  and  negative  at  the  other.  These 
ends  are  called  electrodes.  That  connected  with  the 
zinc,  or  with  the  metal  most  acted  on,  is  called  the 
negative  electrode,  and  that  with  the  copper,  or  with 
the  metal  least  acted  on,  the  positive  electrode. 

322.  Amalgamation  of  the  Zinc.  — Ordinary  zinc 
is  impure,  and  is  violently  acted  on  by  the  liquid  when 
the  circuit  is  broken.  This  both  wastes  the  zinc  and 
the  liquid,  and  weakens  the  strength  of  the  current 
when  the  circuit  is  completed.  It  may  be  remedied 
by  cleaning  the  zinc  by  dipping  it  in  acid  water,  and 
then  rubbing  some  mercury  over  its  surface : the  zinc 
is  then  said  to  be  amalgamated.  The  zinc  is  partially 
dissolved  by  the  mercury,  and  brought  in  a pure  state 
to  the  surface  of  the  plate.  A number  of  voltaic  cells, 
arranged  so  that  the  direction  of  the  current  is  the 
same  in  all,  is  called  a voltaic  battery. 

323.  Electro-Motive  Force — Resistance. — By  the 

electro-motive  force  of  a voltaic  battery  we  mean  the 


CURRENT  ELECTRICITY. 


275 


force  wliicli  produces  or  causes  the  electric  current,  or 
the  force  which  urges  it  forward.  The  electro-motive 
force  varies  with  the  hind  of  metals  and  liquids  em- 
ployed. The  electro-motive  force  of  voltaic  electricity 
is  very  much  less  than  that  of  frictional  electricity ; 
so  that  the  power  of  voltaic  electricity  to  jump  across 
any  non-conducting  material  separating  the  electrodes 
— as,  for  example,  air- — is  almost  nothing. 

The  quantity  of  electricity  which  flows  through  a 
conductor  in  any  given  time  is  called  the  current. 

Anything  which  opposes  the  passage  of  the  current 
is  called  a resistance. 

The  poorer  the  conducting  power  of  any  substance, 
the  greater  the  resistance  it  opposes  to  the  passage  of 
the  current ; the  greater  the  conducting  power,  the  less 
the  resistance. 

In  conductors  of  the  same  material,  the  resistance 
increases  with  the  length  of  the  conductor,  and  de- 
creases with  the  area  of  its  cross-section.  Thus,  a 
copper  wire  of  the  same  thickness,  but  twice  the 
length  of  another,  has  twice  the  resistance.  A cop- 
per wire  of  the  same  length  as  another,  but  twice  the 
area  of  cross-section,  has  but  half  the  resistance. 

Copper  is  about  six  times  a better  conductor  than 
iron ; an  iron  wire  of  the  same  length  and  thickness 
as  one  of  copper  would,  therefore,  have  six  times  the 
resistance  as  the  copper,  and  placed  in  the  circuit  of 
a battery  would  allow  less  current  to  flow  through  it 
than  it  would  were  it  of  copper.  A copper  wire  could 
I be  six  times  as  long  as  an  iron  wire  of  the  same  thick- 
I ness,  and  yet  have  no  greater  resistance. 

I Liquids  are  extremely  poor  conductors  of  electricity. 
{ Acidulated  water  is  several  million  times  a poorer 
conductor  than  pure  copper.  The  resistance  of  the 


276 


NATURAL  PUILOSORHY. 


liquid  between  tbe  plates  of  a voltaic  battery  is,  there- 
fore, very  raucli  greater  than  that  of  a comparatively 
long  wire  joining  them.  The  larger  the  plates  of  a 
battery,  and  the  nearer  they  are  together,  the  less  is 
the  resistance  of  the  mass  of  liquid  between  them. 

The  smaller  the  resistances  of  the  liquid  in  a battery,  and  that  of 
anything  placed  in  the  circuit  of  the  conductors  outside  the  battery, 
the  greater  is  the  current  which  can  flow  through  the  circuit. 

In  joining  the  separate  cells  of  a voltaic  battery,  so  that  the  current 
may  flow  in  the  same  direction  in  each,  if  we  join  the  positive  elec- 
trode of  one  cell  to  the  negative  electrode  of  the  next,  and  so  on,  we 
increase  the  resistance  of  the  liquid  conductor,  because  we  increase  its 
length.  Such  an  arrangement  is  spoken  of  as  a high-resistance  battery 
or  connection  in  series.  If,  however,  we  connect  all  the  positive  elec- 
trodes of  the  different  cells  by  one  wire,  and  all  the  negative  electrodes 
by  another,  and  then  join  their  separate  wires,  we  decrease  the  liquid 
resistance,  because  we  increase  its  area  of  cross-section  by  increasing  the 
size  of  the  plates.  Such  an  arrangement  is  called  a low-resistance  bat- 
tery or  connection  in  multiple  arc. 


324.  Varieties  of  the  Voltaic  Cell. — Volta’s  orig- 
inal battery  consisted  of  discs  of  copper,  clotli,  and  zinc 
moistened  with  acid- water,  piled  on  eacli  other  in  the 
following  order,  viz. : copper,  cloth,  ziuc,  copper,  cloth, 
zinc,  etc.  This  was  called  the  voltaic  pile.  It  has  now 
been  very  greatly  improved. 

There  are  a great  many  forms  of  voltaic  cells,  but 
they  may  all  be  arranged  under  two  classes,  viz. : 

1st.  Those  in  which  but  a single  liquid  is  used  ; and, 
2d.  Those  in  which  two  different  liquids  are  used.  In 
this  case,  oue  of  the  metals  dips  into  one  of  the  liquids, 
and  the  other  metal  into  the  other. 

Among  the  most  important  of  the  single-fluid  bat- 
teries are  Smee's  and  the  Bichromate  Battery ; and  of 
the  double-fluid  batteries,  DanielTs.  the  Gravity.  Grove's., 
and  Bunsen's  Battery. 


CURRENT  ELECTRICITY. 


Til 


325.  Smee’s  Battery.  — The  metals  consist  of  a 
plate  of  silver,  the  surface  of  which  is  coated  with  plati- 
num in  a finely  divided  state,  and  a plate  of  zinc.  The 
metals  are  dipped  into  water  containing  sulphuric  acid. 

326.  The  Bichromate  Battery.  — Zinc  forms  one 
of  the  plates,  and  a plate  of  the  hard  carbon,  or  graphite, 
that  is  formed  inside  of  gas  retorts,  the  other.  Some- 
times a single  plate  of  zinc  is  placed  between  two  plates 
of  carbon.  The  plates  are  immersed  in  a liquid  con- 
sisting of  a substance  called  potassium  bichromate,  dis- 
solved in  water  containing  sulphuric  acid.  The  liquid 
at  first  is  bright  red,  but  after  being  used  changes  to  a 
greenish  brown. 

327.  Daniell’s  Battery.  — The  metals  are  zinc  and 
copper,  and  the  liquids  are  water  containing  sulphuric 
acid,  and  a saturated  solution  in  Avater  of  copper  sul- 
phate. The  zinc  dips  into  the  acid  water  placed  in 
a cell  of  unglazed  eartheuAvare,  called  the  porous  cell.i 
placed  inside  a larger  jar,  Avhich  contains  the  solution 
of  copper  sulphate.  The  copper  is  placed  around  the 
porous  cell  in  the  form  of  a cylinder.  Near  the  top 
of  the  copper  cylinder  is  placed  a small  cage  A\dth  a 
perforated  bottom.  This  cage  is  kept  filled  Avith  crys- 
tals of  copper  sulphate,  and  is  so  placed  as  to  be  partly 
covered  by  the  liquid ; by  this  means  the  strength  of 
the  solution  is  maintained. 

328.  The  Gravity  Battery  is  a modification  of  Dan- 
iell’s,  but  dispenses  AAUth  the  porous  cell.  The  zinc  is 
hung  near  the  top  of  a cell  containing  Avater  above  and 
a solution  of  copper  sulphate  beloAV.  The  copper  plate 
is  placed  in  the  bottom  of  the  cell,  and  has  crystals  of 
copper  sulphate  placed  on  it.  The  AAure  attached  to  the 
copper  plate  is  insulated  by  Avax  or  India-rubber.  After 

24 


278 


NATURAL  PHILOSOPHY. 


the  action  of  the  cell  has  begun,  the  zinc  plate  is  sur- 
rounded. bj  a solution  of  zinc  sulphate,  and  the  copper 
bj  a solution  of  copper  sulphate : the  solutions  being 
of  different  densities,  are  thus  kept  separated. 

The  Daniell’s  and  the  Gravity  Batteries  give  constant 
currents  of  electricity;  the  latter  is  now  almost  uni- 
versally used  on  telegraph  lines. 

329.  Grove’s  and  Bunsen’s  Batteries. — In  Grove’s 
Battery  the  metals  are  zinc  and  platinum,  and  the  liquids 
sulphuric  acid  in  water  and  strong  nitric  acid.  The 

platinum  is  dipped  into  the 
nitric  acid  contained  in  a por- 
ous cell,  and  the  zinc,  in  the 
form  of  a cvlinder,  is  dipped 
in  the  acid  water  contained  in 
a larger  cell. 

In  Bunsens  Battery  the 
metals  are  zinc  and  carbon. 
The  carbon  dips  into  nitric 
acid  contained  in  a porous  cell, 
Pig.  143.— The  Nitric  Acid  Battery,  and  the  ziuc  as  before,  in  the 
form  of  a cylinder,  into  water  and  sulphuric  acid  in  an 
outer  cell. 

Both  of  these  batteries  give  very  intense  currents, 
which,  however,  are  not  constant.  They  are  some- 
times called  nitric  acid  batteries. 

330.  Thermo-Electricity. — If  two  bars  of  anv  un- 
like metals,  as,  for  example,  copper  and  iron,  or  anti- 
mony and  bismuth,  be  soldered  together  at  one  end, 
and  the  other  ends  be  connected  by  a wire,  or  any  other 
conductor,  and  the  soldered  end  heated,  a current  of 
electricity  will  flow  through  the  circuit  so  provided, 
from  the  bismuth  to  the  antimony,  and  through  the 


SYLLABUS. 


279 


wire,  or  other  conductor,  back  again  to  the  bismuth. 
If  the  soldered  end  be  cooled,  a current  of  electricity 
will  also  be  produced,  but  in  the  opposite  direction, 
that  is,  from  the  antimony  to  the  bismuth.  Such  an 
arrangement  is  called  a thermo-electric  couple. 

Currents  of  electricity  produced  in  this  way  by  the 
action  of  heat,  are  called  thermo-electric  currents.^  and 
will  continue  to  flow  as  long  as  there  is  any  difference 
of  temperature  between  the  opposite  ends  of  the  bars. 

Thermo-electric  currents  are  in  general  of  but  very 
feeble  intensity.  Their  intensity  varies  with  the  kind 
of  metals  used,  and  within  certain  limits,  with  the  dif- 
ference of  temperature  between  the  opposite  ends  of 
the  bars.  The  intensity  or  electro-motive  force  may 
be  considerably  increased  by  the  same  means  as  those 
employed  with  voltaic  batteries,  viz. ; by  the  use  of  a 
number  of  thermo-electric  couples  suitably  connected. 
In  this  case  a number  of  bars  of  two  unlike  metals, 
such,  for  example,  as  antimony  and  bismuth,  or  iron 
and  copper,  are  soldered  together  at  their  alternate  ends. 
Such  an  arrangement  is  called  a thermo-pile. 

Syllabus. 

There  are  three  principal  sources  of  electrical  current,  viz. : chemical 
action,  heat,  and  the  motion  of  magnets. 

Electricity  produced  by  chemical  action  is  called  voltaic  electricity. 
Galvani  ascribed  the  convulsive  twitchings  of  the  frog’s  legs  to  a vital 
fluid : Volta  ascribed  these  movements  to  electricity. 

A voltaic  cell  consists  of  two  dissimilar  metals,  immersed  in  a liquid 
capable  of  acting  on  one  of  the  metals,  and  connected  outside  the  liquid 
by  a metallic  conductor. 

The  electricity  produced  in  a voltaic  cell  passes  through  the  liquid 
from  the  metal  most  acted  on  to  the  metal  least  acted  on,  and  out  into 
the  liquid  from  the  metal  least  acted  on,  back  again  to  that  most  acted 
on,  thus  moving  in  a circuit  called  the  voltaic  circuit. 


280 


NATURAL  PHILOSOPHY. 


When  the  conducting-wire  connecting  the  plates  outside  of  the  liquid 
is  broken,  a charge  of  opposite  electricities  collects  at  the  broken  ends  ; 
these  ends  are  called  electrodes. 

The  electro-motive  force  is  the  force  which  causes  the  electricity,  or 
that  which  urges  it  to  flow.  Its  flow  is  opposed  by  the  resistances  of 
the  materials  through  which  it  has  to  pass. 

The  resistance  of  the  liquid  conductor  between  the  plates  of  a bat- 
tery can  be  decreased  either  by  bringing  the  plates  nearer  each  other, 
or  by  increasing  the  size  of  the  plates  immersed. 

Voltaic  batteries  can  be  divided  into  single-fluid  batteries  and  double- 
fluid  batteries. 

The  principal  single-fluid  batteries  are  Smee’s  and  the  Bichromate ; 
the  principal  double-fluid  batteries  are  Daniell’s,  the  Gravity,  Grove’s, 
and  Bunsen’s.  Thermo-electric  currents  are  those  produced  by  heat. 

Questions  for  Review. 

Name  the  principal  sources  of  electrical  current. 

Define  voltaic  couple;  voltaic  cell.  What  is  the  source  of  voltaic 
electricity  ? 

Describe  briefly  the  discoveries  of  Galvani  and  Volta. 

Describe  a simple  voltaic  cell.  What  is  a voltaic  battery? 

What  is  meant  by  a voltaic  circuit?  What  is  meant  by  making  or 
completing  the  circuit?  What  by  breaking  the  circuit? 

Describe  the  polarity  of  the  simple  voltaic  cell.  Define  electrodes. 

What  is  meant  by  the  amalgamation  of  the  zinc? 

What  is  electro-motive  force?  Upon  what  does  it  depend? 

Name  the  resistances  which  are  present  in  every  voltaic  battery. 

Name  all  the  circumstances  which  decrease  the  resistance  of  any  con- 
ductor. 

In  what  two  ways  may  the  resistance  of  the  liquid  conductor  be- 
tween the  plates  of  a voltaic  battery  be  diminished?  Distinguish 
between  a connection  of  a number  of  battery  cells  in  series  and  in 
multiple  arc. 

Into  what  two  classes  may  all  the  different  forms  of  voltaic  cell  be 
arranged?  Describe  each  of  the  following  cells,  viz.:  Smee’s,  the 
Bichromate,  Daniell’s,  the  Gravity,  Grove’s,  and  Bunsen’s. 

What  do  you  understand  by  thermo  electric  currents  ? How  may 
these  currents  be  developed? 

Define  thermo-electric  couple ; thermo-electric  pile. 


CHAPTER  V. 

PROPERTIES  OF  AN  ELECTRICAL  CURRENT. 

331.  Effects  Produced  by  an  Electrical  Current.— 

The  passage  of  an  electrical  current  through  a wire  or 
other  conductor,  produces  in  the  wire  or  other  con- 
ductor a number  of  effects,  the  principal  of  which  are 
as  follows,  viz. : 

1st.  Thermal  effects.  — The  wire  becomes  heated. 

2d.  Luminous  effects. — If  the  wire  be  broken  at  any 
point,  a brilliant  flash  of  light  appears. 

3d.  Physiological  effects. — An  electrical  current  sent 
through  the  body  of  an  animal  produces  involuntary 
movements  of  the  muscles. 

4th.  Chemical  effects.  — An  electrical  current  sent 
through  a compound  liquid  conductor  causes  a de- 
composition and  recombination  of  its  constituent  ele- 
ments. 

5th.  Magnetic  effects.  — All  conductors  conveying 
electrical  currents  are  thereby  rendered  magnetic,  that 
is,  acquire  the  property  of  attracting  or  repelling  bod- 
ies called  magnets. 

332.  Thermal  Effects. — Whenever  a definite  vol- 
taic current  flows  through  a conductor,  it  heats  the  con- 
ductor. The  elevation  of  temperature,  in  the  case  of 
a wire,  is  almost  inappreciable  if  the  wire  be  stout  and 

I 24*  281 


282 


NATURAL  PHILOSOPHY. 


of  good  conducting  material,  unless  tlie  current  be 
very  great.  If,  however,  the  wire  be  fine,  so  as  to  offer 
a great  resistance.,  it  may  become  intensely  heated  or 
even  melted  by  the  current,  if  the  latter  is  sufficiently 
great. 

333.  Luminous  Effects.  The  Voltaic  or  Elec- 
tric Arc. — When  a conductor  conveying  the  current 
from  a powerful  battery  is  broken  at  any  point,  a brill- 
iant flash  of  light  is  seen.  If  the  ends  of  the  wire 
are  connected  with  two  pencils  of  hard  carbon,  or  some 
other  material,  and  brought  together,  and  then  slowly 
separated,  a brilliant  arc  or  light,  called  the  voltaic  arc, 
will  continue  to  pass  between  the  electrodes,  unless 
they  be  too  widely  separated.  The  light  of  the  voltaic 
arc  is  of  dazzling  brightness,  and  the  arc  itself  one  of 
the  most  intense  sources  of  heat  that  can  be  produced 
artificially. 

AVhen  the  carbon  electrodes  are  separated  from  each 
other,  portions  of  the  positive  electrode  are  volatilized 
by  the  current  of  electricity  and  carried  through 
the  air  to  the  negative  electrode,  thus  forming  a 
bridge  of  vapor  over  which  the  electricity  passes. 
The  positive  carbon  decreases  in  size  and  the  negative 
carbon  increases.  Besides  this,  the  carbons  being  in- 
tensely heated,  are  gradually  consumed  by  ordinary 
combustion. 

334.  Illumination  by  the  Electric  Light. — The 

intense  brilliancy  of  the  electric  light  renders  it  ad- 
mirably adapted  for  the  illumination  of  light-houses, 
or  large  buildings,  or  the  streets  of  cities. 

The  consumption  of  the  carbon  electrodes  by  com- 
bustion, and  the  growth  of  the  negative  carbon  at  the 
expense  of  the  positive,  render  it  necessary  to  adopt 


PROPERTIES  OF  AN  ELECTRIC  CURRENT.  283 


Fig,  144. 
The  Jab-' 
locbkoff 
Candle. 


some  means  bj  'vvbicb  the  carbons  may  he  Tcept  a con- 
stant distance  apart;  for,  if  they  sliould  get  too  far 
apart,  the  current  at  once  ceases,  and  tlie  light  goes  out, 
in  which  case  the  carbons  must  be  brought  to- 
gether again,  and  slowly  separated  before  the 
light  reappears.  The  carbons  are  kept  at  the 
same  distance  apart  by  various  forms  of  regu- 
lators. One  of  the  simplest  of  these  regulators 
is  a late  contrivance,  called  the  Jahlochhoff 
candle.  It  consists  of  two  carbon  pencils,  A 
and  i?.  Fig.  141,  placed  parallel  to  each  other, 
and  separated  by  some  non-conducting  mate- 
rial, such  as-  pure  clay  or  alumina,  or  plaster 
of  Paris.  As  the  ends  of  the  carbons  are  con- 
sumed, the  material  separating  the  pencils  is  fused 
and  volatilized,  thus  expos- 
ing fresh  carbons  for  con- 
sumption. 

The  intense  brilliancy  of 
the  electric  light  renders  it 
impossible  to  directly  ex- 
amine the  carbon  electrodes 
between  which  the  arc  is 
passing.  Colored  glasses, 
which  cut  off  most  of  the 
light,  may  be  employed  for 
this  purpose.  A more  sat- 
isfactory way  is  to  form  a 
magnified  image  of  the  car- 
bon electrodes,  by  means 
of  a suitable  lens  placed  in  front  of  them.  The  image 
so  formed  is  received  by  a distant  screen.  Fig.  145 
represents  an  image  so  obtained.  If  a small  piece  of 
any  metal,  such,  for  example,,  as  copper  or  silver,  is 


Fig.  145. 

An  Image  of  the  Carbon  Electrodes. 


284 


NATURAL  PHILOSOPHY. 


placed  ou  the  positive  electrode,  it  is  at  once  volatilized 
and  carried  over  in  the  form  of  vapor. 

335.  Physiological  Effects. — An  electrical  current 
passed  through  the  nerves  of  a recently  killed  animal, 
causes  convulsive  movements  in  the  muscles  that  are 
connected  with  these  nerves.  Passed  through  the  nerves 
of  the  living  animal,  it  produces  vmrious  physiological 
actions,  many  of  which  are  favorable  to  the  cure  of  cer- 
tain diseases.  Electricity,  however,  as  a curative  agent, 
may  do  more  harm  than  good,  and  should  never  he  em- 
gjloyed  except  by  a skilful  and  intelligent  physician. 

336.  The  Chemical  Effects.  Electrolysis. — -TYhen 
a voltaic  current  is  passed  through  any  compound  sub- 
stance in  the  liquid  condition,  it  decomposes  the  sub- 
stance — one  of  its  elements,  or  sets  of  elements,  appear- 
ing at  one  of  the  electrodes,  and  the  other  at  the  other 
electrode.  This  decomposition  is  called  electrolysis. 
W hen  two  elements  combine  chemically  with  each  other, 
one  is  considered  to  be  electro-positive  and  the  other  elec- 
tro-negative. When  a substance  undergoes  electrolysis,  the 
electro-positive  element  appears  at  the  negative  electrode, 
and  the  electro-negative  element  at  the  positive  electrode. 
In  salts  of  the  metals,  the  metal  is  electro-positive,  and 
the  element  or  elements  Avith  Avhich  it  is  combined  are 
electro-negative. 

337.  Electrolysis  of  Water.  Electro-Metallurgy. 

— If  two  platinum  strips  be  made  the  electrodes  of  a 
voltaic  battery,  and  plunged  into  water  Avhich  has  been 
rendered  slightly  acid  for  the  purpose  of  increasing  its 
conducting  poAver,  the  current  in  passing  through  the 
Avater  Avill  decompose  it,  and  hydrogen  Avill  be  giA'en 
off  at  the  negative  electrode  and  oxygen  at  the  posi- 
tive. If  the  electrodes  be  dipped  into  a solution  of  any 


PROPERTIES  OF  AN  ELECTRIC  CURRENT.  285 


salt  of  a metal,  as  copper  sulphate,  the  passage  of  the 
! current  will  decompose  the  salt,  and  metallic  coi^per 
will  appear  at  the  negative  electrode,  and  sulphuric 
V'  acid  and  oxygen  will  he  set  free  at  the  positive  electrode. 

\ If  the  positive  electrode  he  of  copper,  instead  of  sulphuric 
acid  and  oxygen  being  set  free,  sulphate  of  copper  will 
I , he  formed,  and  thus  keep  up  the  strength  of  the  solution. 
In  this  way  we  can  deposit  strong  adherent  films  of 
metal  on  the  surface  of  any  conductor ; for  if  the  article 
to  be  coated  be  attached  to  the  negative  electrode  of  a 
■ battery,  and  dipped  into  a solution  of  the  metal  with 
; which  we  desire  to  coat  the  article,  say  copper,  and  the 
i positive  electrode  be  attached  to  a plate  of  copper,  and 
also  dipped  into  the  liquid,  when  the  current  passes,  the 
salt  will  be  decomposed,  and  the  metal  deposited  in  a 
uniform  layer  over  the  article  at  the  negative  electrode. 
This  process  is  called  electro-metallurgy , and  by  it  arti- 
cles may  be  coated  with  gold,  silver,  copper,  iron,  and 
other  metals. 

The  cuts  in  this  book  are  prepared  as  follows ; They  are  first  cut  in. 
wood  by  a skilful  artist;  the  wood-cuts,  however,  are  not  used  directly 
for  printing,  as  they  are  expensive  and  would  soon  wear  out.  The 
wooden  cuts  are  pressed  into  a mixture  of  soft  wax  and  black-lead, 
and  a perfect  impression  is  thus  obtained.  This  impression  is  covered 
on  the  back  with  some  non-conductor  of  electricity  and  attached  to 
the  negative  electrode  of  a battery,  and  immersed  in  a solution  of 
copper  sulphate.  By  the  passage  of  the  current,  the  wax  mould  is 
thus  covered  with  a thin  sheet  of  copper,  which  is  now  the  exact  coun- 
terpart of  the  figure  on  the  wooden  Mock.  This  film  is  removed  from 
the  wax  mould  and  stiffened  by  being  filled  with  stereotype  metal, 
and  the  form  thus  obtained  is  used  for  printing. 

338.  Magnetic  Effects. — All  conductors,  no  matter 
what  the  nature  of  their  substance,  become  magnetic 
during  the  passage  of  an  electrical  current  through 
them,  and  thereby  acquire  all  the  properties  of  mag- 
nets. Since,  however,  magnetic  properties  are  capable 


286 


NATURAL  PHILOSOPHY. 


of  existing  in  certain  substances,  under  circumstances 
in  wbicli  the  presence  of  electrical  currents  through 
the  magnets  are  not  evident,  we  will  first  discuss  the 
properties  of  such  bodies. 

Bodies  that  are  capable  of  acquiring  magnetic  prop- 
erties under  circumstances  in  which  the  presence  of 
electrical  currents  are  not  evident,  are  called  perma- 
nent-magnets ; those  in  Avhich  magnetic  properties  are 
only  produced  during  the  passage  of  an  electrical  cur- 
rent are  called  electro-magnets.  Hard  iron  and  steel 
are  the  principal  substances  that  can  be  rendered  per- 
manently magnetic.  Any  conductor  of  electricity  can 
become  an  electro-magnet. 


Syllabus. 

The  effects  produced  by  the  flow  of  an  electrical  current  are,  1st. 
Thermal;  2d.  Luminous;  3d.  Physiological;  4th.  Chemical;  and, 
5th.  Magnetic. 

An  electrical  current  traversing  a wire  raises  the  temperature  of  the 
wire.  If  the  wire  be  thin  and  the  current  powerful,  the  wire  will 
become  luminous,  and  may  be  fused. 

If  a wire  conveying  a powerful  current  be  broken  at  any  point  and 
separated  a slight  distance,  a brilliant  flash  of  light  is  seen.  If  two 
pieces  of  hard  carbon  be  connected  to  the  wires  and  then  separated, 
a brilliant  arc,  called  the  voltaic  or  electric  arc,  will  continue  to  pass 
between  them. 

The  light  of  the  electric  arc  is  very  intense,  and  well  adapted  for  the 
lighting  of  large  areas. 

During  the  passage  of  the  current  the  positive  carbon  decreases  in 
size  and  the  negative  increases.  To  maintain  a constant  distance  be- 
tween the  electrodes,  various  devices,  called  regulators  of  the  electric 
light,  are  used. 

Electrical  currents  passed  through  the  bodies  of  dead  animals  cause 
muscular  movements. 

Electricity  should  never  be  employed  as  a curative  agent  except  by 
a skilled  and  intelligent  physician. 


QUESTIONS  FOR  REVIEW. 


287 


The  chemical  effects  of  an  electric  current  are  seen  in  the  combina- 
tions and  decompositions  produced  by  the  passage  of  a current  through 
a compound  substance. 

Electro- metallurgy  is  the  deposit  of  one  metal  on  another  by  the 
action  of  an  electrical  current. 

Conductors  conveying  electrical  currents  become  magnets. 


Questions  for  Review. 

Enumerate  the  different  effects  produced  by  the  passage  of  an  elec- 
trical current. 

How  may  the  heating  effects  of  an  electrical  current  be  shown  ? 

What  is  meant  by  the  voltaic  or  electric  arc?  How  may  it  be  ob- 
tained? What  changes  are  produced  in  the  positive  and  negative 
carbons  during  the  passage  of  the  current  through  the  arc? 

Why  must  the  carbon  electrodes  used  in  the  employment  of  the  elec- 
tric light  for  purposes  of  illumination  be  kept  a constant  distance  apart? 

Describe  the  Jablochkoff  candle.  How  are  the  carbon  electrodes 
maintained  at  a constant  distance  in  this  candle? 

Describe  the  appearance  of  the  image  of  carbon  electrodes  used  for 
producing  an  electric  arc. 

Describe  any  of  the  physiological  effects  of  an  electrical  current. 

What  is  meant  by  electrolysis?  Describe  the  process  of  electro- 
metallurgy. 

Describe  the  magnetic  effects  produced  by  the  passage  of  an  elec- 
trical current  through  a conductor. 

Distinguish  between  permanent-  and  electro-magnets. 


CHAPTER  VI. 

MAGNETISM. 

339.  Natural  Magnets.  — There  exists  in  nature  an 
ore  of  iron  called  magnetic  oxide,  specimens  of  which 
are  sometimes  found,  which,  possess  the  properties  of 
magnets.  Such  magnets  are  called  loadstones,  or  natural 
magnets,  to  distinguish  them  from  those  which  may  be 
obtained  artificially. 

340.  Permanent  Magnets. — When  a piece  of  hard- 
ened steel  is  rubbed  by  a magnet,  the  steel  itself  be- 
comes magnetic.  Steel  can  be  magnetized  in  a variety 
of  ways,  and  as  the  magnets  so  obtained  are  stronger 
than  those  found  in  the  earth,  they  are  preferred  to 
them.  Various  forms  are  given  to  these  artificial  mag- 
nets, but  they  are  generally  made  either  in  the  form 
of  a straight  or  curved  bar. 

341.  Distribution  of  the  Magnetic  Force. — If  some 
iron  filings  be  scattered  over  the  surface  of  a bar- mag- 
net, they  will  be  found  to  collect  mainly  at  the  ends 
of  the  bar,  while  the  middle  will  be  quite  free.  The  ' 
ends  of  the  bar,  or  the  points  where  the  force  is  mani- 
fested with  greatest  power,  are  called  the  poles  of  the 
magnet. 


288 


MAGNETISM. 


289 


A magnetic  needle  consists  of  a magnetized  bar  of  steel, 
made  in  the  form 
shown  in  Fig.  146, 
and  supported  at  its 
centre  of  gravity  on 
a point,  around  which 
it  is  free  to  move. 

When  sucb  a needle 
comes  to  rest,  it  will, 
if  no  other  magnet  is 
near  it,  point  nearlv 
due  north  and  south.  146.  — A Magnetic  Needle. 

That  end  which  points  to  the  north  pole  of  the  earth 
is  called,  in  this  country,  the  north  pole.^  and  the  other 
end  the  south  pole. 

There  are  always  two  opposite  poles  in  every  mag- 
net. Even  if  the  magnet  were  suddenly  broken  in  the 
middle,  the  broken  ends  would  be  found  to  have  polar- 
ity opposite  to  that  of  the  other  ends. 

342.  Attractions  and  Repulsions  of  Magnets.  — If 

the  north  pole  of  a magnet  be  brought  near  the  south 
pole  of  a magnetic  needle,  they  attract  each  other ; but 
if  the  north  pole  be  brought  near  the  north  pole  of  the 
needle,  they  repel  each  other.  We  generally  express 
these  facts  as  follows,  viz. : 

Like  magnetic  poles  attract.^  and  unlike  poles  repel; 
that  is,  north  attracts  south,  or  south  north  ; but  north 
repels  north  and  south  repels  south.  When  magnets 
are  at  a considerable  distance  from  one  another,  the 
force  with  which  they  attract  or  repel  decreases  with 
the  square  of  the  distance  between  them. 

343.  Magnetic  Field.  — Since  magnets  attract  or 
repel  all  other  magnets  brought  near  them,  they  are 

25  T 


290 


NATURAL  PHILOSOPHY. 


supposed  to  be  surrounded  by  an  atmosphere  of  mag- 
netic influence  called  the  macjnetic  field.  The  direction 
of  the  lines  in  which  the  magnetic  force  acts  in  this 
field  is  beautifully  shown  by  the  following 

Experiment.  — Place  a magnet  on  a table,  and  lay  over  it  a piece 
of  smooth  window-glass,  or  a sheet  of  stiff  paper  stretched  in  a frame. 
Sprinkle  some  fine  iron  filings  on  the  glass  or  paper,  and  then  tap  the 
edge  gently  with  the  finger,  and  the  filings  will  be  arranged  in  curved 
lines,  which  are  the  lines  in  which  the  magnetic  force  acts  in  the  mag- 
netic field. 

Experiment. — Magnetize  a large  needle  by  rubbing  its  ends  against 
the  opposite  poles  of  a magnet.  Stick  it  through  a small  piece  of  cork 
large  enough  to  float  it  on  water,  and  place  the  cork  so  that  the  needle 
may  be  parallel  to  the  water.  If  properly  magnetized,  the  needle  will 
j)oint  north  and  south.  Now  approach  to  it  the  poles  of  a magnet, 
and  prove  that  like  poles  repel  and  unlike  poles  attract. 

344.  Magnetism  by  Contact. — When  a bar  of  steel, 
or  other  body  capable  of  being  magnetized,  is  rubbed 
against  a magnet,  it  becomes  itself  magnetic.  Iron, 
steel,  nickel,  cobalt,  manganese,  and  a few  other  sub- 
stances, can  be  magnetized  in  this  way.  Pure,  soft  iron 
is  very  easily  magnetized,  but  only  retains  its  magnet- 
ism while  in  contact  with  the  magnet.  Hardened  steel 
is  not  so  readily  magnetized,  but  retains  its  magnetism 
after  being  removed  from  the  magnet. 

345.  Magnetism  by  Induction. — When  any  body 

capable  of  being  mag- ' 
netized  is  brought 
within  the  magnetic 

.field,  it  becomes  a 
Jfiff.  147.  — Magnetic  Induction.  , 

magnet  without  hav- 
ing actually  touched  the  magnetizing  body.  Magnetism 
produced  in  this  waj'^  is  said  to  be  produced  by  induc- 
tion. The  polarity  of  the  magnetism  produced  in  any 
body  hy  contact  and  by  induction  is  the  same  in  either 


MAGNETISM. 


291 


cose,  and  is  always  of  the  opposite  polarity  to  that  of  the 
magnet  to  which  the  body  is  touched.^  or  near  which  it  is 
brought.  Thus,  suppose  a bar  of  steel  be  touched  to 
the  north  pole  of  a magnet ; the  point  touched  will 
have  south  goolarity  ; or  suppose  the  end  of  a bar,  /S", 
to  be  brought  near  the  north  pjole,  N,  of  the  magnet. 
Fig.  147,  without  touching  it;  then  the  nearer  end 
will,  as  before,  have  south  polarity. 

Experiment.  — Touch  one  end  of  a steel  pen  to  the  north  pole  of  a 
magnet.  Throw  the  pen  in  some  iron  filings,  and  it  will  be  found  to 
have  a pole  both  at  the  end  touched  and  at  the  opposite  end.  By  means 
of  a magnetic  needle,  test  the  polarity  of  the  end  touched,  and  it  will 
be  found  to  be  south,  and  that  of  the  opposite  end  north. 

Experiment.  — Move  one  end  of  a steel  pen  several  times  very  near 
the  north  pole  of  a magnet,  without  actually  touching  it.  Iron  filings 
will  show  poles  at  each  end,  and  the  little  floating  magnetic  needle 
will  show  that  end  which  was  brought  near  the  north  pole  of  the 
magnet  to  be  of  south  polarity,  and  the  opposite  end  north. 

346.  Magnetization  of  Soft  Iron  and  Hardened 
Steel. — Soft  iron  and  hard  steel  differ  greatly  in  the  ease 
with  which  they  become  magnetized.  If  a piece  of 
soft  iron  be  touched  to  the  pole  of  a strong  magnet,  it 
will  at  once  become  magnetized  throughout.  As  soon, 
however,  as  it  is  removed  from  the  magnet,  it  at  once 
loses  its  magnetism.  This  is  not  the  case  with  a piece 
of  hardened  steel.  When  once  magnetized  by  touch- 
ing a magnet,  it  will  retain  its  magnetism  after  separa- 
tion therefrom. 

347.  Electro-Magnets. — When  a current  of  electricity 
is  caused  to  flow  through  any  conductor.!  it  gives  that  con- 
ductor the  properties  of  a magnet. 

If,  for  example,  the  current  flows  through  a copper 
wire,  the  wire  will  acquire  the  property  of  attracting 
iron  filings,  which  it  will  instantly  lose  when  the  circuit 
is  broken. 


292 


NA  TURAL  PHIL  0 SO  PHY. 


If  the  wire  conveying  the  current  be  held  near  a 
magnetic  needle,  it  will  deflect  the  needle.  If  the  wire 
be  held  above  and  parallel  to  the  needle,  it  will  deflect 
the  north  pole  in  one  direction  ; but  if  it  be  held  below 
the  needle,  it  will  deflect  the  north  pole  in  the  opposite 
direction. 

In  order  to  obtain  strong  magnets  by  electrical 
currents,  a considerable  length  of  copper  wire,  covered 
with  cotton,  silk,  or  some  other  insulating  substance, 
is  wrapped  in  the  shape  of  a coil  around  a bar  of  soft 
iron,  as  seen  in  Fig.  1-18.  When  an  electric  current  is 
sent  through  such  a coil,  and  the  coil  thereby  becomes 
magnetic,  it  strongly  magnetizes,  by  induction,  the  soft- 
iron  core  within  it.  The  strength  of  the  magnetism  so 
produced  is  very  much  greater  than  that  of  the  coil 
without  any  soft-iron  core. 

Magnets  so  obtained  differ  in  no  respect  from  per- 
manent magnets,  except  that  they  retain  their  proper- 
ties only  during  the  passage  of  the  current. 

348.  Methods  of  Magnetization.  — Bodies  capable 
of  receiving  magnetism,  may  be  magnetized  hy  touch, 
by  induction,  and  by  electrical  currents. 

Merely  touching  the  end  of  a needle  or  penknife  to 
the  pole  of  a powerful  magnet  will  render  it  magnetic 
throughout;  but  the  needle  or  knife  can  be  more  pow- 
erfully magnetized  by  drawing  it  a number  of  times 
from  its  centre  to  the  end  over  one  of  the  poles  of  a 
magnet,  being  careful  to  return  the  bar  each  time 
through  the  air,  and  to  make  the  stroke  always  in 
the  same  direction.  Then  put  the  middle  of  the 
needle  or  knife  over  the  other  pole  of  the  magnet, 
and  rub  towards  the  opposite  end  in  the  same  manner. 

Sometimes  two  magnets  are  placed  with  their  oppo- 


MAGNETISM. 


293 


site  poles  in  the  middle  of  the  bar  to  be  magnetized, 
and  moved  to  the  ends  in  opposite  directions,  and  then 
returned  through  the  air  and  stroked  as  before. 

By  far  the  most  powerful  magnets,  however,  are  pro- 
duced by  means  of  electrical 
currents.  The  electrical  cur- 
rent is  caused  to  flow  through 
a hollow  coil  of  wire  in  which 
is  placed  the  bar  to  be  mag- 
netized ; or,  as  is  most  fre- 
quently the  case,  the  current 
is  used  to  excite  magnetism 
in  an  electro  - magnet,  and 
the  bar  to  be  magnetized  is 
rubbed  against  the  poles  of  the  electro-magnet  so  pro- 
vided, as  shown  in  Fig.  148. 

349.  Cause  of  the  Needle  Pointing  to  the  North. 

— The  magnetic  needle  points  to  the  north  pole  of  the 
earth  for  the  same  reason  that  the  opposite  poles  of 
magnets  point  to  each  other,  if  they  are  sufficiently 
near  and  free  to  move.  The  earth  acts  as  a huge  mag- 
net.^  with  its  magnetic  poles  in  the  neighborhood  of  the 
poles  of  the  earth and  the  magnetic  needle  points 
towards  these  poles  on  account  of  their  attraction. 

Since  it  is  the  opposite  poles  of  magnets  that  attract  each  other,  the 
end  of  the  needle  that  points  towards  the  north  pole  of  the  earth  must 
be  of  the  opposite  polarity  to  the  earth's  polarity  at  the  north.  The 
French,  for  this  reason,  call  the  end  of  the  needle  which  points  to- 
wards the  north  pole  of  the  earth,  the  austral  or  south  pole. 

350.  Origin  of  the  Earth’s  Magnetism.  — The 

origin  of  the  earth’s  magnetism  is  not  exactly  known, 
though  there  is  no  doubt  that  it  is  in  some  manner  con- 
nected with  the  sun’s  action.  It  is  quite  probable  that 
the  principal  causes  are  induction  from  the  sun,  and 
25* 


Fig.  148. 

Magnetism  hy  Electro-Magnets. 


294 


NATURAL  PHILOSOPHY. 


electrical  currents  developed  in  tlie  earth  in  a variety 
of  ways,  though  perhaps  chiefly  by  heat  and  chemical 
action. 

351.  The  Declination  or  Variation  of  the  Needle. 

— It  is  a very  common,  though  mistaken  notion,  to  sup- 
pose that  the  magnetic  needle  invariably  points  to  the 
true  geographical  north.  The  fact  is  that  except  in  a 
few  localities,  it  actually  points  to  the  east  or  west  of 
the  true  north.  This  deviation  of  the  needle  from  the 
true  north,  is  called  the  declination  or  variation.,  and 
in  some  localities  amounts  to  a considerable  deviation. 

352.  The  Inclination  or  Dip  of  the  Needle. — 

When  a magnetic  needle  is  free  to  move  in  a vertical 
as  well  as  a horizontal  direction,  it  remains  in  but  few 
parts  of  the  earth  in  a true  horizontal  position.  In 
most  places  one  of  the  poles  is  inclined  or  dipped  to- 
wards the  earth.  This  is  called  the  dip  or  inclination 
of  the  needle.  In  the  northern  hemisphere  it  is  the 
north  pole,  and  in  the  southern  hemisphere  the  south 
pole  that  is  inclined. 

The  cause  of  the  dip  is  the  greater  pull  or  attraction 
of  one  of  the  poles  of  the  earth  on  the  needle  than  the 
other,  so  that  the  needle  is  pulled  down  to  the  earth  as 
well  as  directed  to  the  north.  Thus,  in  the  northern 
hemisphere,  the  north  pole  of  the  needle  being  nearer 
the  earth’s  magnetic  pole  than  the  south  pole  of  the 
needle,  is  pulled  down  or  dipped,  the  dip  being  greater 
the  nearer  the  needle  is  to  the  earth’s  magnetic  pole. 
When,  however,  the  needle  is  midway  between  the 
poles  it  remains  horizontal,  because  the  attraction  of 
the  opposite  poles  of  the  earth  is  equal. 


SYLLABUS. 


295 


Syllabus. 

A magnet  is  a body  that  possesses  tlie  power  of  attracting  and  re- 
pelling other  magnets.  Magnets  are  either  natural  or  artificial. 

If  iron  filings  be  sprinkled  on  a bar-magnet,  they  will  all  collect  at 
the  ends  of  the  bar,  leaving  the  middle  free  from  attracted  particles. 
Those  ends  where  the  greatest  amount  of  filings  collect,  or  where  the 
magnetic  force  is  greatest,  are  called  poles.  There  are  always  at  least 
two  opposite  poles  in  every  magnet. 

A magnetic  needle  consists  of  a small  magnetized  bar,  supported  at 
its  centre  of  gravity  so  as  to  be  free  to  move.  When  such  a magnet 
comes  to  rest  it  points  nearly  due  north  and  south ; the  end  which 
points  towards  the  north  pole  of  the  earth  is  called  the  north  pole 
of  the  needle,  the  other  end  is  called  the  south  pole. 

Like  poles  of  magnets  repel  one  another,  unlike  poles  attract. 

The  magnetic  field  is  the  atmosphere  of  magnetic  influence  surround- 
ing the  poles  of  a maguet. 

When  a body  receives  magnetism  from  another  by  being  rubbed  or 
touched  with  it,  it  is  said  to  be  magnetized  by  contact ; when  it  re- 
ceives magnetism  from  another  body  by  being  brought  into  its  mag- 
netic field,  it  is  said  to  be  magnetized  by  induction. 

klagnetism  by  contact,  or  by  induction,  is  always  of  the  opposite 
polarity  to  that  of  the  body  giving  the  magnetism. 

Bodies  capable  of  being  magnetized,  may  receive  their  magnetism 
by  touch,  by  induction,  and  by  electrical  currents.  The  latter  produce 
the  most  powerful  magnetism. 

The  magnetic  needle  points  to  the  north  pole  of  the  earth  because 
the  earth  acts  like  a huge  magnet,  with  its  poles  near  the  north  and 
south  geographical  poles ; the  magnetic  poles  of  the  earth  attract  the 
poles  of  the  needle,  and  cause  them  to  point  towards  them. 

The  earth’s  magnetism  is  most  probably  caused  mainly  by  inductiop 
from  the  sun,  and  electrical  currents  in  the  earth. 

The  needle  does  not,  in  the  majority  of  places  on  the  earth,  point  to 
the  true  north,  but  to  the  east  or  the  west  of  it.  This  deviation  is 
called  the  deelination  or  variation. 

When  the  ends  of  a magnetic  needle  are  free  to  move  in  all  direc- 
tions, in  most  places  one  of  them  dips  or  inclines  to  the  earth.  This  is 
called  the  dip  or  inclination  of  the  needle. 


296 


NATURAL  PHILOSOPHY. 


Questions  for  Review. 

What  are  loadstones?  Distinguish  between  natural  and  artificial 
magnets. 

How  is  the  magnetic  force  distributed  in  a magnetic  bar  ? Define 
magnetic  poles.  How  many  poles  must  there  be  in  every  magnet? 
What  names  are  given  to  these  poles? 

Describe  the  magnetic  needle.  State  the  law  of  magnetic  attrac- 
tions and  repulsions. 

How  may  these  laws  be  experimentally  verified?  How  may  a very 
simple  magnetic  needle  be  constructed  ? 

Define  magnetic  field.  How  may  the  directions  in  which  the  mag- 
netic force  acts  in  a magnetic  field  be  ascertained  ? Distinguish  be- 
tween the  manner  of  producing  magnetism  by  contact  and  by  induc- 
tion. 

If  a bar  of  steel  be  touched  by  the  north  pole  of  a magnet,  what 
will  be  the  polarity  produced  in  the  steel  at  the  point  touched?  If 
the  end  of  a bar  of  steel  be  brought  near  the  north  pole  of  a magnet, 
what  polarity  will  be  produced  at  that  end? 

Describe  the  methods  of  producing  magnetism  by  contact.  Describe 
the  method  usually  adopted  for  producing  magnetism  by  electrical 
currents.  What  is  an  electro-magnet? 

Why  does  the  magnetic  needle  point  nearly  to  the  geographical 
north?  What  is  the  probable  cause  of  the  earth's  magnetism? 

Define  declination  or  variation  of  the  magnetic  needle.  Define 
inclination  or  dip.  By  what  is  the  inclination  or  dip  of  the  needle 
caused  ? 


0 


CHAPTER  VII. 

MAGNETO-ELECTRIC  CURRENTS.— APPARA- 
TUS DEPENDENT  ON  ELECTRO-MAGNETS. 

353.  The  Galvanometer.  — In  order  to  ascertain 
whether  an  electrical  current  is  flowing  through  anj 
conductor,  it  is  only  necessary  to  bring  near  it  a mag- 
netic needle,  and  observe  whether  or  not  the  needle  is 
deflected. 

Unless,  however,  the  current  is  powerful,  the  needle, 
even  if  delicate,  is  not  visibly  deflected.  In  order  to 
magnify  the  effect  of  the  electrical  current,  it  is  caused 
to  pass  through  an  instrument  called 
a galvanometer. 

The  galvanometer  is  an  instrument 
used  to  detect  the  presence  of  elec- 
trical currents,  and  to  measure  their 
intensity.  It  consists  of  a length 
of  copper  wire  insulated  by  being 
wrapped  with  silk  or  cotton,  and 
wound  in  the  form  of  a flat  ring  or 
helix,  a.  Fig.  149.  A magnetic 
needle  is  suspended  inside  the  coil  by  a fibre,  5,  of  silk. 
The  current  is  sent  through  the  coil  by  making  it 
enter  at  one  of  the  binding-posts,  x or  ?/,  and  pass  out 
at  the  other.  Since  each  turn  of  the  wire  becomes 
magnetic  during  the  passage  of  the  current,  it  is  evi- 

297 


298 


NAT  UR  A L PHIL  OSOPIIY. 


dent  that  an  increase  in  the  uumher  of  turns  will  cause 
an  increase  in  the  attraction  or  repulsion  which  the 
coil  has  for  the  magnetic  needle. 

Before  using  the  galvanometer,  the  coil  is  placed  so 
that  the  direction  of  the  wire  is  parallel  with  the  needle, 
that  is,  the  coil  is  placed  with  the  wire  extending  in  a 
north  and  south  direction.  On  the  passage  of  the  cur- 
rent, the  needle  is  deflected  so  as  to  tend  to  he  placed 
at  right  angles  to  the  direction  in  which  the  current  is 
flowing;.  The  streng:th  of  the  current  is  then  deter- 
mined  from  the  deflection  of  the  needle. 

354.  Induction  by  Current  Electricity. — We  have 
seen  that  permanent  magnets  are  capable  of  producing 
magnetic  properties  in  suitable  substances  placed  near 
them,  that  is,  they  are  capable  of  producing  magnetisjn 
by  induction. 

AYe  have  also  seen  that  electro-magnets  possess  all 
the  properties  of  permanent  magnets.  AY e would  sup- 
pose, therefore,  that  electro-magnets  should  also  possess 
the  property  of  causing  magnetism  by  induction,  and 
this,  as  Ave  have  also  seen,  is  the  case. 

But  electrical  currents  passing  through  conductors 
render  them  magnetic  ; therefore  we  might  suppose 
that  magnets  should  be  capable  of  producing  elec- 
trical currents.  That  electrical  currents  can  be  so 
produced  Avas  first  proved  by  the  illustrious  Faraday. 

355.  Production  of  Electrical  Currents  by  the  In- 
duction of  other  Electrical  Currents.  — AYheuever 
an  electrical  current  begins  to  flow^  or  ceases /to  floA\', 
through  a conductor,  it  causes,  by  induction,  electrical 
currents  in  neighboring  conductors.  The  currents  so 
produced  are  said  to  be  due  to  the  induction  of  the 
current  floAving  through  the  first  conductor. 


MAGNETO-ELECTRIC  CURRENTS. 


299 


The  induced  currents  continue  but  for  short  inter- 
vals, that  is,  they  only  flow  at  the  moment  of  making 
or  of  breaking  a circuit. 

The  presence  of  induced  currents  produced  in  this 
manner  can  be  shown  by  means  of  the  apparatus  seen 
in  Fig.  150.  A hoUow  coil,  A,  of  moderately  stout,  in- 


Fig.  150.  — Indnctioa  by  Cnrrent  Electricity. 


sulated  Avire,  called  the  primary  coil,  is  connected  by 
wires,  a,  fc,  c,  cZ,  Avith  a battery -cell,  C.  Another  holloAV 
coil,  B,  called  the  secondary  coil,  formed  of  a consider- 
able length  of  insulated  wire,  surrounds  the  primary 
coil.  The  ends  of  this  coil  are  connected  by  means 
of  the  Avires  e and  / to  a galvanometer,  G. 

If,  noAV,  one  of  the  wires  conveying  the  battery- 
current,  as  d,  be  raised  from  the  mercury  in  the 
cup,  B,  so  as  to  break  the  circuit,  that  is,  to  cause 
the  electricity  to  stop  floAving  through  the  primary 
coil,  a momentary  induced  current  Avill  at  once  be  pro- 
duced in  the  secondary  coil,  as  will  be  shown  by  the 
ijiovement  of  the  needle  of  the  galvanometer  in  a cer- 
tain direction.  After  a moment  the  needle  Avill  come 
to  rest,  thus  showing  that  the  current  in  the  secondary 
coil  has  ceased  to  floAv.  If,  noAV,  the  Avire  d be  again 
placed  in  the  mercury  cup,  so  that  a current  from 
the  battery  can  begin  to  fioAV  through  the  primary 


300 


NATURAL  PHILOSOPHY. 


coil,  the  galvanometer  needle  will  again  be  deflected, 
but  in  the  opposite  direction  to  that  produced  by  the 
breaking  of  the  circuit  of  the  primary  coil,  thus  show- 
ing that  the  induced  current  produced  in  the  secondary 
coil,  by  making  contact  with  the  battery-circuit,  is  in 
the  opposite  direction  to  that  produced  by  breaking 
the  contact. 

It  can  be  shown  that,  at  the  moment  of  breaking  the 
circuit  of  the  primary  coil,  the  current  is  flowing  through 
the  secondary  coil  in  the  same  direction  as  the  current  in 
the  primary  coil,  and  that  at  the  moment  of  mahing  the 
contact  it  is  flowing  in  the  opposite  direction.  The  former 
is  sometimes  called  a direct,  and  the  latter  an  inverse 
current.  The  current  producing  the  induction  is  some- 
times called  primary  current,  and  the  current  which 
is  induced,  the  secondary  current. 

It  is  only  at  the  moment  of  making  or  of  breaking 
the  primary  current  that  the  secondary  current  is  in- 
duced ; tluit  is,  the  induced  current  is  only  produced 
while  the  intensity  of  the  primary  current  is  either  in- 
creasing or  decreasing . 

356.  Induction  Currents  Produced  by  the  Move- 
ment of  the  Primary  Current. — If,  while  the  current  is 
still  flowing  through  the  primary  coil,  the  latter  be  drawn 
out  or  away  from  the  secondary  coil,  the  intensity  of  its 
influence  on  the  secondary  coil  will  gradually  decrease, 
and  a direct  current  of  short  duration  will  be  induced  in 
the  secondary  coil.  If  the  primary  be  pushed  within  or 
towards  the  secondary,  the  intensity  of  its  influence  on 
the  secondary  will  gradually  increase,  and  an  inverse 
current  of  short  duration  will  be  induced  in  the  secondary. 

357.  Currents  Produced  by  Magnets  — Magneto- 
Electricity. — If  the  ends  of  the  secondary  coil  be  con- 


ELECTRO- MAGNETIC  CURRENTS. 


301 


nected  with  the  galvanometer,  and  a permanent  bar- 
magnet  be  thrust  into  the  coil,  a momentary  current  is 
produced,  as  is  shown  by  the  deflection  of  the  needle.  If 
the  magnet  is  now  drawn  out  of  the  coil,  a momentary 
current  is  also  produced,  but  in  the  opposite  direction 
to  the  first,  as  is  seen  by  the  movement  of  the  galvan- 
ometer needle.  Currents  produced  by  the  motion  of 
magnets  are  called  currents  of  magneto-electricity . They 
will  also  be  produced  if  the  magnet  remains  stationary 
and  the  coil  is  moved. 

358.  Production  of  Electricity  from  Power  — 
Dynamo-Electric  Machines, — Very  powerful  elec- 


Fig.  161.  — The  Gramme  Dynamo-Electrio  Machine. 

trical  currents,  similar  to  those  obtained  from  large 
voltaic  batteries,  can  be  obtained  directly  from  me- 
26 


302 


NATURAL  PHILOSOPHY. 


clianical  power  by  means  of  dynamo-electric  machines. 
In  these  machines,  by  means  of  a steam-engine  or  other 
source  of  power,  a number  of  coils  of  unre,  called  the 
armature.^  are  set  into  rapid  revolution  between  the 
poles  of  powerful  electro-magnets.  The  currents  so 
produced  in  the  armature  are  caused  to  take  the 
same  direction  by  means  of  a contrivance  called  the 
commutator. 

A variety  of  dynamo-electric  machines  are  now  con- 
structed. Fig.  151  shows  a very  successful  form  known 
as  the  Gramme  Machine.  The  field-magnets  are  shown 
at  M M and  M'  M\  and  the  poles  of  the  magnets  at  A 
and  A,  which  are  north  and  south  poles  respectively. 
The  armature  is  seen  at  A,  and  the  commutator  at  (7. 

The  coils  on  the  armature  and  those  on  the  field-magnets  are  con- 
nected in  the  same  circuit,  so  that  on  first  starting,  the  current  pro- 
duced in  the  armature  coils,  flowing  through  the  coils  of  the  field-mag- 
nets, makes  them  stronger.  The  stronger  magnetic  field  so  produced 
reacts  on  the  armature  coils,  and  thereby  increases  the  intensity  of  the 
currents  produced  therein. 

Dynamo-electric  machines  are  now  very  successfully 
employed  for  the  production  of  electricity  for  illumi- 
nation and  for  electro-plating. 

359.  Applications  of  Electro-Magnets. — The  power 
possessed  by  electro-magnets  of  retaining  their  magnet- 
ism only  during  the  passage  of  the  electrical  current, 
enables  them  to  be  employed  in  a great  variety  of  elec- 
trical apparatus. 

360.  The  Electro-Magnetic  Telegraph.  — Electro- 
magnetic telegraphy  depends  for  its  operation  on  the 
fact  just  mentioned,  that  a bar  of  soft  iron  can  be  in- 
stantly magnetized  and  de-magnetized  by  making  and 
breaking  the  circuit  of  a helix  of  wire  wrapped  around 
the  iron. 


ELECTRO- MAGNETIC  CURRENTS. 


303 


There  are  many  diftereut  systems  of  telegraphy. 
That  most  generally  used  in  this  country  is  called 
the  Morse  system,  after  the  name  of  its  inventor. 

In  the  Morse  system  a battery.,  a hey^  and  a Morse  in- 
strument are  placed  in  an  electrical  circuit,  at  a distant 
portion  of  which  is  another  key  and  Morse  instrument. 
The  key  consists  of  an  arrangement  by  means  of  which 
the  circuit  may  be  easily  made  or  broken.  The  Morse 
instrument  consists  of 
an  electro-magnet,  i/, 

Fig.  152,  with  a piece 
of  soft  iron,  J.,  called 
an  armature  or  keeper., 
attached  to  one  end  of 
a bar,  B,  pivoted  at  C. 

The  other  end  of  this 

bar  is  furnished  with 

• , ,7  Ti  Fig.  152,  — A Morse  Eeceiving  Instnunent. 

a point  or  stylus,  B,  ^ ^ 

which  presses  against  a long  strip  of  paper,  S,  moved 
by  clock-work  under  the  point.  If  the  operator  at  the 
distant  station  should  depress  his  key  when  the  circuit 
is  broken,  he  thereby  closes  it,  and  M,  becoming  mag- 
netic, pulls  down  the  armature.  A,  and  causes  the  stylus, 
P,  to  indent  the  paper.  If  he  keeps  the  key  down  for 
some  little  while,  the  paper  will  be  drawn  over  the 
stylus,  and  a dash  or  long  mark  made  on  it ; but  if 
the  key  is  only  kept  down  for  a moment,  then  but  a 
dot  will  be  given  to  the  paper.  When  the  operator 
raises  the  key,  the  circuit  is  broken,  M ceases  to  be  a 
magnet,  and  the  point  is  pulled  away  from  the  paper 
by  the  spring,  Q.  By  adopting  a system  of  dots  and 
dashes  to  represent  the  letters  of  the  alphabet,  commu- 
nications can  be  carried  on  very  rapidly  between  dis- 
tant places. 


304 


NATURAL  PHILOSOPHY. 


Skilled  operators  soon  learn  to  readtke  dispatcli  from 
tlie  sound.  In  most  offices  the  paper  instrument  just  de- 
scribed is  replaced  by  an  instrument  called  the  sounder., 
similar  in  construction  to  the  other,  but  dispensing 
with  the  paper  and  clock-work. 

361.  The  Morse  Alphabet. — In  the  following  table 
are  given  the  combinations  of  dots  and  dashes  employed 
in  the  Morse  system  to  represent  the  letters  of  the  al- 
phabet and  the  numerals. 


a -—  i s --- 

b k t - 

c --  - 1 u 

d m V 

e - n — - w 

f o - - X 

g — p y — 

h q z ---  - 

i --  r - --  & - --- 

1 6 

2  7 

3  8 

4  9 

6 0 


To  avoid  the  running  of  one  letter  into  another,  such  as  t - - and 
e - , which  might  be  mistaken  for  - - - s,  or  - - - c,  a space  is  left 
between  successive  letters  longer  than  that  between  any  of  the  sepa- 
rate dots  of  any  single  letter,  and  a still  longer  space  is  left  between 
words. 

It  will  be  noticed  that  two.  dots,  an  interval,  and  one  dot  stand  for 
c,  while  three  dots  stand  for  s;  f is  represented  by  two  dots,  while  o is 
represented  by  a dot,  an  interval,  and  a dot.  Similar  differences  are 
noticed  in  the  signs  for  h and  y,  c and  r,  2 and  d-.  These  differences 
are  more  marked  when  only  the  sounds  are  regarded,  and,  indeed. 


ELECTRO-MAGNETIC  CURRENTS. 


305 


most  telegraphy  by  the  Morse  system  is  effected  by  means  of  the  sounds, 
as  already  explained. 

362.  The  Magneto-Electric  Telephone  is  an  in- 
strument b}'"  means  of  which  the  sounds  of  the  human 
voice,  as  in  articulate  speech  uttered  in  any  place,  can 
be  audibly  reproduced  at  places  hundreds  of  miles  dis- 
tant. This  wonderful  instrument  is  the  invention  of 
an  American  citizen  by  the  name  of  Bell. 

The  telephone  consists  of  a permanent  magnet,  d/. 
Fig.  153,  with  a e a l 


it'  a 


B H 

The  Magneto-Electric  Telephone  Circnit. 


coil,  (7,  of  insulat- 
ed wire  wrapped 
around  it  near  one 
of  its  ends.  One 
end  of  this  coil 
is  connected  to  a wire,  E X,  which  passes  to  a dis- 
tant station,  where  it  is  connected  to  one  end  of  the 
coil.  O',  wrapped  around  the  permanent  magnet,  M'. 
The  other  ends  of  the  coils  C and  C are  either  con- 
nected by  means  of  a wire  between  A B H.,  or,  Avhat 
is  the  same  thing,  are  connected  to  metallic  plates 
buried  in  the  earth  at  A and  H.  The  circuit  so  pro- 
vided is  called  the  telephone  circuit.  The  apparent 
break  in  the  wire,  E X,  at  a,  is  intended  to  represent  a 
great  length  of  wire. 

The  method  of  operation  of  the  instrument  is  as  fol- 
lows : A circular  diaphragm,  X,  of  thin  sheet-iron  is  fast- 
ened at  its  edges  to  a mouth-piece,  P,  of  wood.  When 
a person  speaks  into  this  mouth-piece,  the  diaphragm 
is  moved  in  and  out  by  the  sound-waves  striking  it ; 
but  the  soft-iron  diaphragm,  X,  being  near  the  magnet, 
JX,  becomes  a magnet  by  induction  ; and  as  it  is  moved 
towards  and  from  the  magnet,  iX,  by  the  sound-waves, 
it  produces  induced  electrical  currents  in  the  coil  of 
26*  U 


306 


NATURAL  PHILOSOPHY. 


wire  on  if,  and  these  currents  traversing  the  circuit, 
flow  through  the  coil  on  if',  at  the  distant  station,  and, 
by  changing  the  strength  of  the  magnetism  of  if', 
cause  the  diaphragm,  f)',  to  pass  through  movements 
precisely  similar  to  those  produced  by  the  sound-waves 
in  D.  An  ear,  therefore,  placed  at  P'  will  distinctly 
hear  all  that  is  said  at  P,  even  though  P'  be  hundreds 
of  miles  distant  from  P.  One  can  even  recognize  the 
peculiarities  of  the  distant  speaker’s  voice. 

The  manner  in  which  the  currents  flowing  through 
the  coil  on  if'  cause  movements  in  the  diaphragm,  P', 
can  be  readily  understood  by  considering  a single  mo- 
tion of  the  diaphragm,  P,  towards  and  from  if.  Sup- 
pose that,  by  moving  towards  if,  it  causes  an  electrical 
current  which,  traversing  the  telephone  circuit  and  flow- 
ing through  the  coil,  C",  increases  the  magnetism  of  if '; 
the  diaphragm,  P',  is  at  once  drawn  nearer  if'.  When, 
now,  P moves  away  from  if,  it  produces  a current  of 
electricity  in  the  opposite  direction  to  that  produced 
by  its  first  movement,  which  current  flowing  through 
C',  decreases  the  magnetism  of  if',  when  the  elasticity 
of  the  diaphragm,  P',  causes  it  at  once  to  move  away 
from  if'. 

Since  these  movements  of  the  diaphragm  correspond 
precisely  to  the  movements  of  the  sound-waves,  it  will 
be  seen  that  the  diaphragm,  P',  is  moved  in  preciselv 
the  same  manner  as  the  diaphragm  of  a person’s  ear 
would  be  if  he  were  near  the  speaker.  A person  list- 
ening at  P'  should,  therefore,  be  able  to  hear  all  that 
is  spoken  at  P. 

It  will  be  seen  that  the  magneto-electric  telephone 
is  in  reality  a magneto-electric  machine  operated  by 
the  voice.  The  sound-waves  striking  the  diaphragm 
produce  electrical  currents  ichich,  flowing  through  the 


ELECTRO- MAGNETIC  CURRENTS. 


307 


telephone  circuit.^  reproduce  in  a distant  diaphragm  the 
exact  movements  of  the  first. 

Electrical  currents  then^  and  not  sound-waves,  pass 
through  the  wire  connecting  the  telephones. 

The  construction  of  the  magneto- 
electric  telephone  will  be  better  under- 
stood by  an  inspection  of  Fig.  154,  which 
represents  a section  of  the  instrument. 

The  magnet,  H F,  has  a coil,  C,  at  one 
of  its  ends,  the  ends  of  the  coil  being 
connected  to  binding  screws  at  x and  y. 

The  diaphragm,  D,  is  placed  near  that 
end  of  the  magnet  which  is  surrounded 
by  the  coil.  The  centre  of  the  dia- 
phragm, D,  is  directly  opposite  the 
opening  in  the  mouth-piece,  P.  The  Bell  Telephone. 


363.  The  Microphone  is  a variety  of  telephone  by 
means  of  which  faint  sounds  can  be  heard  at  very  great 
distances.  The  microphone  was  invented  by  Professor 
Hughes. 

In  this  instrument  the  sound-waves  are  made  to  vary 
the  electrical  resistance  of  a circuit,  and  thereby  pro- 
duce variations  in  the  quantity  of  electricity  which 
flows  through  the  circuit.  These  variations,  by  means 
of  a telephone,  are  made  to  reproduce  the  sounds  caus- 
ing them.  The  microphone  is  placed  near  the  place 
where  the  sounds  are  originated,  and  connected  by 
means  of  a conducting  wire  with  a battery  and  a tele- 
phone. Whatever  sounds  are  made  near  the  micro- 
phone, such  as  talking  or  even  breathing,  can  be  heard 
by  any  one  listening  at  the  telephone,  even  though  it 
be  hundreds  of  miles  distant. 

Fig.  155  shows  the  construction  of  the  microphone. 


308 


NATURAL  PHILOSOPUY. 


A small  rod,  A 7i,  of  hard  carbon,  sharpened  at  both 


ends,  is  loosely  placed 
in  small  cavities  in 
two  other  carbons, 
C and  supported, 
as  shown,  by  a piece 
of  thin  Avood,  A,  at- 


tached to  the  base, 
S.  The  Avires,  a and 
^ 7,  connected  to  the 

W ends  of  C and  A,  are 


placed  in  the  circuit 
of  a voltaic  battery. 


Fig.  165.  — The  Microphone. 


in  Avhich  is  also  placed  a magneto-electric  telephone. 

The  amount  of  electricity  from  the  battery  that  can 
floAV  through  the  microphone  will  be  dependent  on  the 
position  of  the  upright  carbon,  A B.  If  it  rest  against 
the  horizontal  carbons,  C and  1),  so  that  a comparatively 
large  extent  of  the  surfaces  of  its  sharpened  ends  are  in 
contact  thercAvith,  more  electricity  Avill  floAv  through 
the  microphone  than  if  a smaller  extent  of  its  surfaces 
Avere  in  contact  Avdth  C and  D.  Any  Amriations  in  the 
strength  of  the  current  flowing  through  the  micro- 
phone, by  producing  Amriations  in  the  strength  of  the 
telephone  magnet,  Avill  cause  movements  in  the  dia- 
phragm of  the  telephone.  If,  uoav,  a person  talks  in  a 
faint  tone  near  the  microphone,  the  sound-Avaves  strik- 
ing different  parts  of  the  instrument — mainly  the  board, 
E — Avill  cause  movements  of  the  carbons,  C and  A, 
Avhereby  the  portions  of  A A that  are  in  contact  with 
C and  D are  A’aried  in  exact  accordance  with  the 
movements  of  the  sound-waA'es ; so  that  a person 
listening  at  the  telephone  Avill  be  able  to  hear  dis- 
tinctly all  that  is  said. 


ELECTRO-MAGNETIC  CURRENTS. 


309 


So  -wonderfully  sensitive  is  this  instrument  that  the  ticking  of  a 
watch  held  near  it  can  be  heard  by  a person  at  the  telephone.  Even 
a fly  walking  over  the  board,  E.  can  be  heard  at  the  distant  telephone. 
The  sound  of  the  voice  as  in  speaking  can  be  most  distinctly  heard  if 
the  board,  E,  is  inclined  at  an  angle  of  about  50°  to  a horizontal.  The 
joint  at/ permits  of  such  inclination. 

364.  The  Phonograph. — The  discovery  of  the  tele- 
phone directed  the  attention  of  many  experimenters  to 
the  mechanical  work  which  the  voice  is  able  to  per- 
form, and  soon  residted  in  the  invention  of  an  appa- 
ratus for  recording  the  sounds  of  the  voice,  and  repro- 
ducing them  at  any  future  time.  This  instrument  is 
called  the  phono gi'cq:ih^  and  was  invented  by  Mr.  Edi- 
son. 


Pig.  156.  — The  Speaking  Phonograph, 


The  construction  of  the  phonograph  is  shown  in  Fig.  156.  A cylin- 
der, C,  is  supported  at  X and  E on  an  axis,  A,  so  as  to  be  readily 
turned  by  means  of  the  handle,  TT.  At  one  of  the  supports,  as  at  A, 
a screw-thread  is  cut  on  the  axis.  This  thread  fits  into  a correspond- 
ing thread  in  the  support  at  X,  so  that  one  complete  turn  of  the  handle, 
ir,  causes  the  C3dinder  to  move  forward  the  distance  between  any 
two  consecutive  threads.  A screw-thread  of  the  same  pitch  is  cut  on 
the  surface  of  the  cylinder,  C.  A mouth-piece,  I/,  attached  to  a cir- 
cular diaphragm,  E,  of  elastic  metal,  similar  to  the  mouth-piece  of  the 
magneto-electric  telephone,  is  connected  to  the  upright  support,  S.  A 
piece  of  watch-spring,  Q,  fastened  at  its  lower  end,  is  placed,  as  shown 
in  the  figure,  near  the  diaphragm,  D,  but  prevented  from  touching  it 
by  means  of  a small  piece  of  rubber  tube,  R.  A needle-point,  P,  is 
soldered  to  the  rod,  Q,  near  its  upper  end. 


310 


NATURAL  PHILOSOPHY. 


In  using  the  instrument,  the  surface  of  the  cylinder,  C,  is  covered 
with  a smooth  piece  of  tin-foil,  and  the  mouth  piece  is  placed  so  that 
the  needle-point,  P,  just  touches  the  tin-foil  surface  directly  over  one 
of  the  grooves  of  the  thread.  If,  now,  a person  talk  in  a loud  voice 
into  the  mouth-piece,  M,  the  sound-waves,  striking  the  diaphragm, 
move  it  in  and  out ; and  this  motion  being  imparted  to  the  needle- 
point, P,  causes  it  to  indent  the  tin-foil  on  the  surface  of  the  cylinder. 
Were  the  cylinder  at  rest,  these  indentations  would  all  be  received  on 
the  same  point ; but  if  the  cylinder  be  kept  constantly  turning  while 
the  person  is  speaking,  the  indentations  are  made  on  different  portions 
of  the  surface. 

During  this  time  the  cylinder  has  been  moved  so  that  the  mouth- 
piece is  now  in  a different  position  from  that  it  occupied  when  the 
person  first  began  to  talk.  To  make  the  phonograph  talk  back,  move 
the  support,  S,  so  that  the  mouth-piece  assumes  the  position  shown  in  the 
figure  at  S'M'  by  the  dotted  lines,  and  reverse  the  motion  of  the  handle, 
W,  until  the  cylinder  is  again  in  the  position  it  occupied  when  the  person 
began  to  talk.  Now  move  the  mouth-piece  back,  so  that  the  point,  P, 
again  touches  the  foil  directly  over  the  groove,  and  again  turn  the 
handle  in  the  same  direction  in  wliich  it  was  turned  when  the  person 
began  to  talk.  If  the  ear  be  now  pilaced  near  the  mouth-piece,  or,  still 
better,  if  a large  paper  cone  be  inserted  in  the  opening,  M,  all  that  was 
spoken  to  the  instrument  will  be  distinctly  repeated  by  it. 

The  cause  of  the  sound  is  as  follows:  The  movements  of  the  point, 
P,  produced  by  words  spoken  into  M,  caused  the  surface  of  the  tin- 
foil  on  C to  be  marked  with  minute  elevations  and  depressions ; and 
when  the  point  is  again  caused  to  pass  over  these  irregularities,  it  re- 
produces in  the  diaphragm,  D,  all  the  movements  by  which  the  irregu- 
larities were  originally  caused ; for  when  the  point  is  forced  to  climb 
any  of  the  elevations  on  the  foil,  it  moves  the  diaphragm  outwards; 
and  when  it  is  forced  to  enter  the  depressions,  the  diaphragm,  D,  moves 
inwards.  A person  near  M will  therefore  hear  all  that  was  origi- 
nally spoken  into  the  phonograph. 

Syllabus. 

In  order  to  ascertain  whether  an  electrical  current  is  flowing  through 
any  conductor,  it  is  only  necessary  to  bring  the  conductor  near  a mag- 
netic needle,  and  observe  whether  the  needle  is  deflected  or  not. 

Galvanometers  are  used  to  detect  the  presence  of  a current  of  elec- 
tricity, or  to  measure  its  intensity.  They  consist  essentially  of  a long. 


QUESTIONS  FOR  REVIEW. 


311 


insulated  wire,  wrapped  in  the  form  of  a hollow  helix,  with  a magnetic 
needle  inside  the  helix. 

A conductor  conveying  a current  of  electricity  will  induce  a mo- 
mentary current  of  electricity  in  a neighboring  conductor,  whenever 
the  intensity  of  the  current  is  increasing  or  diminishing. 

If  a magnet  be  moved  towards  or  from  a coil  of  wire,  it  will  pro- 
duce a current  of  electricity  in  the  wire. 

In  dynamo-electric  machines,  powerful  electric  currents  can  be  pro- 
duced by  the  movements  of  electro-magnets  between  the  poles  of  other 
electro-magnets. 

The  electro-magnetic  telegraph  depends  for  its  operation  on  the  fact 
that  a core  of  soft  iron,  surrounded  by  a coil  of  insulated  wire,  can  be 
instantly  magnetized  and  de-magnetized  by  completing  or  breaking  an 
electrical  current,  in  the  circuit  of  which  the  coil  is  placed. 

In  the  magneto-electric  telephone  the  sound-waves,  striking  a me- 
tallic diaphragm  placed  near  a permanent  magnet  surrounded  by  a 
coil  of  insulated  wire,  produce  electrical  currents  in  the  coil,  which, 
flowing  through  a telephone  circuit,  reproduce  in  a distant  diaphragm 
the  exact  movements  of  the  first  diaphragm. 

The  microphone  is  a variety  of  telephone,  by  means  of  which  faint 
sounds  can  be  heard  at  very  great  distances. 

The  phonograph  is  an  instrument  by  means  of  which  the  sounds  of 
the  voice,  as  in  speaking,  may  be  recorded  and  reproduced  at  any 
future  time. 

Questions  for  Review. 

How  can  we  ascertain  whether  an  electrical  current  is  flowing  through 
a conductor  ? 

Describe  the  construction  and  use  of  the  galvanometer. 

What  is  meant  by  induced  electrical  currents  ? How  may  such  cur- 
rents be  developed  ? Are  such  currents  continuous  or  momentary  ? 

What  are  inverse  currents?  What  are  direct  currents? 

How  may  induced  currents  be  produced  by  permanent  magnets  ? 

What  are  dynamo-electric  machines  ? Describe  their  general  con- 
struction. 

Describe  the  general  construction  and  method  of  operation  of  the 
Morse  Telegraphic  Instrument. 

Explain  in  full  the  construction  and  method  of  operation  of  the 
magneto-electric  telephone. 

What  is  the  use  of  the  microphone  ? Describe  its  construction  and 
method  of  operation.  Describe  the  construction  of  the  phonograph. 


INDEX. 


A FACE 

Absorption  of  heat 187 

of  light 216 

selective 188 

Adhesion 84 

between  liquids 85 

between  liquids  and  gases 90 

between  solids 85 

between  solids  and  gases 89 

between  solids  and  liquids 86 

Adhesion,  influence  of  on  boiling- 

point 201 

varieties  of. 85 

Affinitt/,  or  chemical  attraction...  84 

Air,  buoyancy  of 134 

Air-Pump,  construction  of. 131 

Alcoholometers Ill 

Alphnbet,  Morse’s  telegraphic 304 

Amnlgumntion  of  zinc  of  batter- 
ies  274 

Annenlini/ 91 

Aqueous  humor  of  eye 23G 

Archimedes’  principle,  experi- 
mental proof  of. 105, 106 

principle  of 105 

Artesiun  well 104 

Athermnnous  bodies 189 

Atmosphere 127 

composition  of 127 

diffusion  of  gaseous  ingredients 

of 128 

greater  mass  of,  near  earth’s  sur- 
face  13.5 

height  of 1‘27 

pressure  of 128 

AtmosjAieric  electricity 264 


P.tGE 


Atmospheric  pressure,  amount 

per  square  inch 129 

pressure,  illustrations  of 132 

pressure,  influence  of  on  liquid 

state 32 

pressure,  simple  experiments 

in 132,  133 

Atoms 19 

Attraction  of  gravitation,  causeof 

unknown 65 

of  gravitation,  effect  of  di.stance 

on 67 

of  gravitation,  effect  of  mass  on..  66 
Attractions  and  repulsions,  elec- 
trical  2.52 

and  repulsions  of  excited  bodies. 

cause  of 2.57 

and  repulsions  of  magnets 289 

B 

Balance,  use  of  in  determining 

specific  gravity 108 

Balloons 134 

Barometer 129 

construction  of 130 

tests  of  accuracy  of. 130 

use  in  measuring  height  of  moun- 
tains  1-30 

use  of  as  a weather-glass 130 

Batteries,  double-fluid 276 

single-fluid 276 

Battery,  bichromate 277 

Bunsen’s 278 

Daniell’s ffii 

Grove’s 278 


312 


INDEX. 


313 


PAGE 


Battery,  high  resistance 276 

low  resistance 276 

Smee’s 277 

the  Gravity 277 

A’oltaic 27-t 

Beaut  of  light 212 

BoUiuy  of  liquids,  laws  of 199 

point,  circumstances  influencing  199 
point,  influence  of  adhesion  on..  201 

Boiler,  steam-engine 201 

JJoff/e,  specific  gravity 110 

BriHlettesx 91 

Banseti  Burner,  how  to  make 181 

photometer 221 

Buoyancy,  centre  of 106 

of  air 134 

of  liquids 105 

c 

Calorimeter 203 

Camera,  the  photographing 238 

obscura 239 

Candle,  Jablochkoff 283 

Capillarity 87 

familiar  instances  of 88 

phenomena,  cau.se  of 88 

phenomena  of 88 

Carbon  electrodes,  image  of 283 

Cau.xe  and  effect 12 

of  color 244 

of  musical  sounds 155 

of  winds 177 

Centiyrade  thermometer  scale....  173 

Centre  of  buoyancy 106 

of  curvature  of  mirror 226 

of  gravity 68 

of  gravity,  bodies  supported  at, 

at  rest 70 

of  gravity,  method  of  determin- 
ing  69 

Centrifugal  force,  so  called 48 

Centripetal  force 48 

Change  of  latent  into  sensible  heat  195 

Charge,  positive  and  negative 254 

Chemical  or  atomic  attraction 84 

Chemistry 11 

Chimneys,  cause  of  draught  in...  177 

Circnit,  electrical 273 

Cohesion 82 

of  leaden  bullets 83 

of  liquids 83 

Cofd  produced  by  evaporation 201 

27 


PAGE 


Color,  cause  of 244 

disc 243 

Comniunication  of  heat 180 

Commutator  of  dynamo-electric 

machines 301 

Compotients  and  resultants 43 

Compound  microscope 239 

Com pressibility  of  liquids 98 

of  matter 21 

Concave  mirrors,  formation  of 

images  by 226 

Condensation  of  vapors  and  gases  34 

Condenser  of  TEpinus 261 

of  steam-engine 206 

Conduction  of  heat 180 

of  heat  by  fluids 182 

Conductivity  of  bodies  for  heat, 

illustrations  of 181 

of  solids,  applications  of 182 

Conductors  of  electricity,  list  of..  251 
Connecting-Bod  of  Steam-engine  205 
Connection  of  voltaic  cells  in 

multiple  arc 276 

of  voltaic  cells  in  series 276 

Contraction  of  matter 21 

Convection  of  heat 183 

Cornea 236 

Couple,  thermo-electric 279 

Cranh  of  steam-engine 205 

Cry.stalline  form 94 

lens  of  eye 236 

Crystallization,  force  of 95 

paradox 200 

Current,  electrical,  definition  of.  275 

electricity 250 

' electricity,  induction  of. 298 

electricity,  sources  of 271 

D 

Bead  points  of  steam-engine 206 

Beclination  of  magnetic  needle.  294 

Biatherma nous  bodies 187 

Biffusion  of  gases 127 

of  light 214 

of  liquids 86 

Birective  tendency  of  magnetic 

needle,  cause  of. 293 

Bispersion  of  light 242 

Bistillation 201 

Bistinct  vision,  conditions  requi- 
site for 236 

vision,  limit  of. 234 


314 


INDEX. 


PAGE 

Divisibility  of  matter 19 

Double  fluid  electrical  hypothesis  255 
Drnuyht  in  chimneys,  cause  of...  177 

Ductility 90 

Dynamo-Electric  machines 301 

E 

Ear 157 

iimits  of  audition 1.58 

Eccentric  shaft 206 

Echoes 148 

multiple 149 

Elasticity 93 

how  developed 93 

limits  of 94 

measure  of 94 

Electric  discharge,  effects  of. 263 

light,  illumination  by 282 

Electrical  attractions  and  repul- 
sions, law  of. 253 

charge 249 

charge,  distribution  of 258 

charge,  effects  produced  by 250 

circuit 273 

current,  chemical  effects 284 

current,  effects  of 281 

current,  luminous  effects 282 

current,  magnetic  effects 285 

current,  physiological  effects 284 

current,  thermal  eflects 281 

discharge,  chemical  effects 264 

discharge,  luminous  effects 263 

discharge,  mechanical  effects 264 

discharge,  physiological  effects..  263 

discharge,  thermal  effects 263 

energy,  varieties  of. 249 

hypotheses 255 

machine,  plate 260 

tension 258 

Electricity,  atmospheric 264 

condensation  of 262 

conductors  of 2.51 

current 2.50 

induction  of 2.56 

nature  of 249 

thermo 278 

Electrodes 274 

Electrolysis 284 

Electro-Maynctic  telephone 307 

magnets,  definition  of. 286 

Electro-Metallurgy 284 


PAGE 

Electro-Motive  force 274 

Electrophorus 2.59 

simple,  how  to  make 260 

Electroscope 253 

Elements 10 

Energy  and  force 13 

not  gained  by  machines 54 

Engine,  steam 204 

units  of  measure 17 

Equilibrium,  neutral,  of  bodies 

supported  on  axis 71 

neutral,  of  floating  bodies 107 


of  bodies  resting  on  a flat  sur- 
face  72 

of  bodies  supported  on  an  axis..  71 
of  liquids  in  communicating 


vessels 104 

stable,  of  bodies  supported  on 

an  axis 71 

stable,  of  floatiug  bodies 107 

unstable,  of  bodies  supported  on 

axis 71 

unstable,  of  floating  bodies 107 

Eqttivulent,  .loule’s 204 

Ether,  the  luminiferous 170 

Evaporation,  circumstances  in- 
fluencing  198 

production  of  cold  by 201 

Expansibility  of  gases 126 

of  matter 21 

Expansion  of  gases 176 

of  liquids 175 

of  solids 174 

of  solids,  examples  of 174 

Experiments  in  high-tension 

electricitj' 266,  267,  268 

Extension  or  magnitude 16 

Eye,  the  human 235 

F 

Fahrenheit's  thermometer 173 

bodies,  laws  of 74 

Field,  magnetic 289 

Floating  bodies,  equilibrium  of...  106 

Flow  of  liquids 116 

amount  of,  rule  for  calculating...  117 
of  liquids  through  horizontal 

pipes 118 

velocity  of,  rule  for  calculating..  116 

Fluids,  conduction  of  heat  by 182 

varieties  of. 30 


INDEX. 


315 


PAGE 

Foci,  conjugate,  of  concave  mir- 


rors   226 

Focus,  principal,  of  concave  mir- 
rors  226 

virtual,  of  concave  mirrors 226 

Force  and  energy 13 

centrifugal,  examples  of 49 

centrifugal,  so  called 48 

centripetal 48 

definition  of 37 

direction  of  action  of. 38 

indestructibility  of. 13 

intensity  of. 39 

magnetic  distribution  of 288 

of  crystallization 95 

point  of  application  of 39 

representation  of. 37 

unaffected  by  condition  of  rest 

or  motion .^ 42 

varieties  of 37 

Forces,  direction  of. 44 

molecular 29 

parallel 46,  47 

parallelogram  of. 44,  45,  46 

Form,  crystalline 94 

French  units  of  measure 17 

Ft  •eezitnj  a gradual  process 196 

effect  of,  on  temperature  of  air...  196 

mixtures 196 

Friction,  cause  of. 25 

definition  of 25 

Fusion,  laws  of. 193 

of  solids,  cause  of 193 


o 


PAGE 

Gravity,  force  of. 65 

intensity  of,  determined  by  the 

pendulum 78 

point  of  application  of 68 

specific 108 

H 

Hardening 91 

Hardness 91 

Head  of  liquid 116 

Heat,  absorption  of 187 

cause  of. 170 

communication  of. 180 

conduction  of 180 

convection  of 182, 183 

emission  or  radiation  of. 187 

general  effect  of. 171 

how  caused  by  chemical  combi- 
nation  207 

influence  of,  on  condition  of 

matter 33 

latent 193 

luminous 186 

mechanical  equivalent  of. 203 

obscure 186 

opacity 187 

radiated  in  all  directions 184 

radiation  of 184 

ray 185 

reflection  of 186 

sensible 193 

shadows 185 

specific 202 

transparency 187 

High-Tension  electricity 249 


Galleries,  whispering 150 

Galvani  and  Volta,  experiments 

of. 272 

Galvanometer,  construction  of...  297 

use  of 297 

Gas,  tension  of 126 

Gases,  condensation  of. 34 

expansibility  of 126 

expansion  of. 177 

incoercible.  so  called 34 

nature  of 32 

Gramme  electrical  machine 301 

Gravitation,  law  of 66 

Gravity,  centre  of 68 

direction  of. 68 

effect  pf. 65 


Tinman  eye 235 

eye,  defects  of. 237 

Hydranlics 115 

Hydrodynamics 9S 

Hydrometer Ill 

Hydrostatic  pre.ss 100 

I 

Hhiminated  bodies 211 

Illumination  by  electric  light 282 

Images,  apparent  size  of. 225 

formation  of  by  lenses 234 

formed  by  small  apertures 221 

virtual 223 

Impenetrability  of  matter 18 


316 


IND  EX. 


PACE 

Inclination  or  dip  of  magnetic 


needle 294 

Inclined  plane 59 

efficiency  of  as  a mechanical 

power 60 

Inde.<<trnctibilitij  of  matter  and 

force 13 

Induced  electrical  currents,  char- 
acter of. 300 

electrical  currents,  direct 300 

electrical  currents,  inverse 300 

Induction,  cause  of. 256 

coil,  primary 299 

coil,  secondary 299 

of  current  electricity 298 

of  electricity 256 

of  magnetism 290 

Inertia 22 

effects  produced  by 23 

examples  of 24 

Instruments,  musical 165 

optical 237 

stringed 165 

wind 166 

Intensity  of  light 219 

of  light,  effect  of  distance 

upon 219,  220 

of  light,  how  measured 220 

of  radiant  heat 186 

Interference  oi  sound-waves 165 

Invisihility  of  ray  of  light 216 

Iris 236 

J 

•Tablocbhoff’s  electric  candle 283 

of  water 119 

Joule’s  equivalent 204 

L 

lactometers Ill 

lantern,  magic 238 

Intent  heat  of  ice-water 195 

heat  of  vapors 201 

ioit',  natural 12 

Inu's  of  boiling  of  liquids 199 

of  falling  bodies 74,  75,  76 

of  fusion 193 

of  reflection  of  light 214,  215 

of  refraction  of  light 218 

of  solidification 194 

lenses,  converging 282 

conjugate  foci  of 233 


PAGE 


lenses,  definition  of 231 

diverging,  varieties  of. 232 

foci  of. 232 

formation  of  images  by 234 

forms  of. 232 

images  at  conjugate  foci  of. 235 

principal  focus  of 233 

virtual  focus  of 233,  234 

lever,  arms  of 57 

classes  of 56 

effects  of. .57 

leydenl&t 262 

light,  absorption  of 216 

and  heat,  difference  between 210 

and  radiant  heat,  relations  of....  245 

beam  of 212 

diffusion  of. 214 

dispersion  of. 242 

intensify  of 219 

nature  of 210 

pencil  of 212 

ray  of 212 

refraction  of 217 

sources  of 211 

velocity  of 213 

lightnitig 264 

rods 265 

rods,  circumstances  affecting 

value  of 265 

limit  oi  distinct  vision 234 

Limits  of  audition 158 

liquefaction 34 

liquid  condition  of  matter,  cause 

of 30 

jets  or  veins,  velocity  of  escape  of  1 16 

pressure,  effect  of  density  on 101 

pressure,  effect  of  depth  on 101 

pressure,  equality  of  upward, 
downward,  and  lateral  press- 
ures  101 

pressure  on  base  of  containing 

ves,sel,  how  determined 102 

pressure  on  side  of  containing 

vessel,  how  determined 102, 103 

pressure,  upward,  how  deter- 
mined  103 

solution 86 

Xii/iiirfs,  boiling  of. 199 

compressibility  of 98 

diffusion  of 86 

expansion  of 175 

mobile 31 


INDEX. 


317 


PAGE 

Liquids,  viscid 31 

Loadstone 28S 

Lont/Siffhtedness,  cause  of- 237 

Luminiferous  eth&T 170 

Luminous  bodies 211 

heat 186 

M 

Machine,  definition  of. 52 

Magic  Lantern 238 

Magnetic  attractions  and  repul- 
sions  289 

field 289 

force,  distribution  of. 288 

needle 289 

needle,  declination  of. 294 

needle,  inclination  or  dip  of 294 

needle,  variation  of 294 

Slagnetism,  of  earth,  cause  of. 293 

produced  by  induction 290 

produced  by  contact 290 

Magneto-Electricitg 301 

Magnets,  electro,  how  produced..  291 

electro,  properties  of. 292 

natural 288 

permanent,  how  produced 288 

Magnitude 16 

Malleahilitg 90 

Mariotte’s  law 135 

Mass  and  velocity 39 

definition  of 39 

effect  of  on  attraction 66 

Matter 9 

changes  of 11 

chemical  changes  of. 11, 13 

conditions  of 29 

indestructibility'  of. 13 

physical  change  of 11 

Maximum  density'  of  water,  ef- 
fect of  on  freezing 176 

density  of  water,  temperature  of  176 

tension  of  vapors 198 

Measurement,  units  of 16 

Mechanical  equivalent  of  heat...  203 

powers 52.  54 

JU'erfiMwt  sonorous 145 


Metre 

Microphone 

Microscope,  the  compound 

the  simple 

Mirrors  and  specula 

concave 

27* 


17 

308 

239 

237 

222 

225 


P.AGE 

Mirrors,  curved 225 

plane 222 

Mixture  of  liquids 85 

Mixtures,  freezing 196 

Mobility 22 

Molecular  forces 29 

Molecules 19 

compound 20 

elementary 20 

jTfoHic/ifum,  examples  of 41 

measure  of,  moving  energy' 41 

on  what  dependent 40 

3lorse  receiving  instrument 303 

sounder 304 

telegraph 303 

telegraphic  alphabet 304 

ilftirioH,  curvilinear 43 

perpetual 61 

rectilinear 43 

uniform 42 

uniformly'  accelerated 43 

uniformly  retarded 43 

varied 42 

varieties  of. 42.  43 

Mouth-Pieces 166 

Multiple  echoes 149 

Musical  box 167 

instruments 165 

sounds 155 


N 

Natural  law 12 

phenomena,  cause  of. 37 

philosophy' 11 

Nature  of  light 210 

Near-Sightedness,  cause  of. 237 

Negative  charge 254 

Noises 155 


C) 

Obscure  heat 

Opacity 

Optical  instruments 
Osmose 


P 

Paper-Bag,  water  boiled  in 200 

Paradox,  cnlinary 200 

Pencil  of  light 212 

Pendulum,  the 77 

Pendulums,  laws  of  oscillations 
of 78 


186 

211 

237 

89 


318 


INDEX. 


PAGE 


Penetrating  power  of  telescope..  241 

Penumbra  or  partial  shadow 213 

Permanent  magnets,  definition  of  286 

Perpetual  motion 61 

Phenomenon,  definition  of. 12 

P/ionogrrap/i,  the  speaking 309 

Phosphorescence,  cause  of. 216 

two  distinct  kinds  of 217 

Photographing  camera 238 

Photometers 220 

Bunsen's 221 

Phgsics,  definition  of. 11 

object  of  study  of 13 

Piston  of  steam-engine 205 

Plane  mirrors,  formation  of  ima- 
ges by 223 

Pneumatics 98,  126 

Points,  influence  of  on  charged 

conductors 258 

Poles,  magnetic 289 

Porositg  of  matter 20 

Positive  charge 254 

Press,  hydrostatic 100 

Pressure,  atmospheric 128 

effect  of  on  density  of  gas 135 

effect  of  on  volume  of  gas 135 

of  liquids,  a mechanical  power..  99 

transmission  of,  in  liquids 99 

Princii>le  of  A'elocities 52 

Properties  common  to  gases  and 

liquids 126 

of  matter 16 

Pulley,  the  fixed 59 

the  mOA'able 59 

the  air 131 

the  force,  for  water 137 

the  suction,  for  water 136 

Q 

Quality  of  sounds,  cause  of  differ- 
ences of 160 

Quantity  oi  motion 40 

R 

Padiant  heat  and  light 245 

heat,  intensity  of. 186 

Padlation  of  heat 184 

of  heat,  how  effected 184 

Painbotv 244 

Jtaij  of  heat 185 

of  light 212 

Pcaction  of  escaping  jet 121 


PAGE 

lieaction,  vase 122 

Reeds 166 

Reflecting  toloscoyo 241 

Reflection  of  heat 186 

of  light 214 

of  light,  amount  of. 215 

of  light,  larvs  of ; 214.  215 

of  sound 147 

Refracting  toloioopo 240 

Refraction,  effect  of  on  apparent 

direction 231 

of  light 217 

of  light,  effects  caused  by 219 

of  light,  laws  of 218 

of  sound 151 

Relation  betAveen  absorbers  and 

reflectors  of  heat 188 

Resistaiice,  electrical 274 

Resistances,  Qmd 25 

to  motion 25 

Resonance 149. 162 

experiments  in 163, 164 

Resonators 163 

Resulta nts  and  components. 43 

Retina 236 

Rivers,  velocity  of  Avater  in 118 

s 

5cre?c,  the.  as  a mechanical  power  60 

Selective  absorption 188 

absorption,  cause  of 189 

Senses,  the 10 

Shadows,  cause  of 212 

of  heat 185 

Simple  microscope 237 

voltaic  cell 272 

Single-Fluid  electrical  hypothe- 
sis  255 

Siphon,  the 136 

.Si'rcH,  the 159 

Sive,  limits  of,  of  structures 93 

S/irfe  valve 205 

Small  apertures,  images  formed 

by 221 

Solid  condition  of  matter,  cause 

of. 30 

Sot  idiflcation — 33 

increase  of  volume  during 197 

laAVS  of 194 

of  fused  solids,  cause  of 193 

Solids,  expansion  of. 174 

Solution  of  liquids 86 


INDEX. 


319 


PAGE 


Solution  of  solids  by  liquids 86 

Sonorfrun  medium 145 

Sonnet,  cause  of. 143 

effect  of  distance  on 151 

intensity  of. 156 

mu.sical  and  noisy 155 

not  transmitted  through  vacuum  145 

pitch  of 157 

quality  of. 160 

reflection  of 147 

reflection  of  by  mirrors 150 

refraction  of. 151 

the  characteristics  of 155 

transmission  of. 145 

transmitted  by  elastic  media 146 

use  of  word 140 

velocity  of 147 

Sound-Waves,  amplitude  of,  ef- 
fect of 156 

how  conveyed  to  ear 143 

interference  of 165 

nature  of 143 

Sources  of  light 211 

Speaking  phonograph 309 

trumpet 156 

tubes 151 

Specific  gravity 108 

gravity,  bottle 110 

gravity  of  liquids,  how  to  deter- 
mine  110 

gravity  of  solids  lighter  than 

water,  how  to  determine 108 

gravity,  rule  for  determining 108 

gravity,  table  of. 112 

heat 202 

heat  of  water 203 

Spectrum 242 

colors  of 242 

colors  of,  refrangibility  of. 242 

Specula  and  mirrors 222 

Steam-Chest 204 

Steam-Engine 204 

high  pressure 205 

String  telephone 146 

Structures,  limits  of  the  size  of...  93 

Sublimation 197 

wees,  compound 10 

elementary 10 

Surface  action  of  bodies  on  heat.  186 

Sympathetic  vibrations 160 

vibrations,  examples  of 161 

Synthesis  of  light 243 


T FACE 

Telegraph,  electro-magnetic 302 

Telegraphic  key 303 

Telephone,  electro-magnetic 307 

the  string 146 

Telescope,  penetrating  power  of...  241 

the  refracting 240 

Temper,  drawing  the 91 

Temperature,  definition  of 171 

how  measured 172 

Tempering 91 

Tenacity 92 

influence  of  sectional  area  on 92 

Tension,  electrical 258 

of  gas 126 

of  vapors,  maximum 198 

Thermo-Electricity 278 

Thermometer,  construction  of.....  172 

graduation  of. 173 

scales.  Centigrade  and  Fahren- 
heit  173 

uses  of. 173 

Thermo-File 279 

Thunder,  cause  of 265 

Torricelli’s  experiment  to  sh\)w 

atmospheric  pressure 129 

rule 117 

Trnnslticent  bodies 211 

Transmission  of  sound 145 

of  sound  by  elastic  media 146 

Transparency  of  metals  for  light  211 

Ti-ansparent  bodies 211 

Trtimpet,  ear 157 

speaking 156 

Tube,  capillary 87 

speaking 151 

Tuning-Fork 143 

u 

JTmbra,  or  complete  shadow 212 

Undershot  water-wheel 119 

Undulation,  definition  of. 142 

Units  of  measure,  English 17 

of  measure,  French 17 

V 

Uapoi'ization 197 

Vapors,  condensation  of 34 

formation  of,  in  vacuum 198 

latent  heat  of. 201 

maximum  tension  of. 198 

Variation  of  magnetic  needle 294 

Velocities,  principle  of 52 


320 


INDEX. 


PAGE 

Velocities,  table  of. 40 

Velocity  of  falling  bodies,  effect 

of  mass  on 74 

of  falling  bodies,  effect  of  shape 

of  body  on 74 

of  light 213 

of  sound 147 

of  sound,  effect  of  temperature 

of  air  on 147 

of  water  in  rivers 118 

Vlhrotion,  definition  of 142 

Vibrations,  sympathetic 160 

angle 224 

Vitreous  humor  of  eye 230 

Voltaic  couple 272 

currents 271 

or  electric  arc 282 

pile 276 

Vol atne,  increase  of  during  solidi- 
fication  197 

of  gas,  effect  produced  by  press- 
ure on 135 

W 

Water,  compressibility  of 98 

electrolysis  of. 284 

specific  heat  of. 203 

temperature  of  maximum  den- 
sity of 176 

wheels 119 


PAGE 

Water-Pump,  the  force 137 

the  suction 136 

Wave,  amplitude  of 142 

definition  of. 142 

length  of. 142 

motion,  nature  of 140, 141 

period  or  time  of  vibration  of,,,,.  142 
Wave.s  of  condensation  and  rare- 
faction  144 

Wedge,  examples  of 60 

the,  as  a mechanical  power 60 

Weight,  French  and  English  sys- 
tems of. 65 

French  system,  values  of 66 

Well,  artesian 104 

Wheel  and  axle .58 

Whtfcls,  breast 121 

overshot 120 

turbine 122. 123 

undershot 119 

water 119 

Whispering  galleries l.'O 

Winds,  cause  of. 177 

Work  done  by  machine,  how  to 
determine 54 

X 

Xylophone 167 

z 

Zinc,  amalgamation  of 274 


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use  of  Schools  and  Academies,  by  Prof.  Philip  Lawkence.  j 

Anatomy,  Physiology,  and  Hygiene. 

A Text-Book  for  Schools,  Academies,  Colleges,  and  Families. 

By  Joseph  C.  Maetindale,  M.D. 

First  Lessons  in  Natural  Philosophy. 

For  Beginners.  By  Joseph  C.  Maetindale,  M.D. 

A History  of  the  United  States. 

From  the  Discovery  of  America  to  the  present  time.  By 
Joseph  C.  Maetindale,  M.D. 

The  Young  Student’s  Companion. 

Or,  Elementary  Lessons  and  Exercises  in  Translating  from  1 
English  into  French.  By  M.  A.  Longsteeth. 

Tables  of  Latin  Suffixes. 

Designed  as  an  Aid  to  the  Study  of  the  Latin  Grammar.  By  ; 
Amos  N.  Cueeiee,  A.M.,  Professor  of  Latin  in  the  University  ; 
of  Iowa. 

3000  Practice  Words.  I 

By  Prof.  J.  Willis  Westlake,  A.M.,  State  Kormal  School, 
Millersville,  Pa.  Contains  lists  of  Familiar  Words  often  Mis- 
spelled, Difficult  Words,  Homophonous  Words,  Words  often  , 
Confounded,  Rules  for  Spelling,  etc.  It  is  a book  that  every 
teacher  wants.  Handsomely  hound  in  flexible  cloth,  crimson 
edges. 

4 


In  the  School-Room; 

Or,  Chapters  in  the  Philosophy  of  Education.  Gives 
the  experience  of  nearly  forty  } ears  spent  in  school-room  work. 
By  John  S.  Hart,  LL.D. 

I Meadows’  Spanish  and  English  Dictionary. 

I In  Two  Parts : I.  Spanish  and  English  ; II.  English  and  Span- 
ish. By  F.  C.  ilEADOWS,  A.M.  j 

The  Model  Pocket-Register  and  Grade-Book. 

A Roll-Book,  Record,  and  Grade-Book  combined.  Adapted  to 
all  grades  of  Classes,  whether  in  College,  Academy,  Seminary,  | 
High  or  Primary  School.  Handsomely  bound  in  fine  English  ^ 
cloth,  bevelled  sides,  crimson  edges. 

The  Model  School  Diary. 

Designed  as  an  aid  in  securing  the  co-operation  of  parents.  It 
consists  of  a Record  of  the  Attendance,  Deportment,  Recita- 
tions, etc.,  of  the  Scholar  for  every  day.  At  the  close  of  the 
week  it  is  to  be  sent  to  the  parent  or  guardian  for  his  examina- 
tion and  signature. 

The  Model  Monthly  Report. 

Similar  to  the  Model  School  Diary,  excepting  that  it  is  intended  | 
for  a Monthly  instead  of  a Weekly  report  of  the  Attendance, 
Recitations,  etc.,  of  the  puiiil. 

Manuals  for  Teachers. 

A Series  of  Hand-Books  comprising  five  volumes,  which  it  is 
believed  will  prove  a valuable  contribution  to  the  art  and  sci- 
ence of  Teaching.  Printed  on  the  best  quality  of  calendered 
pajier  and  handsomely  bound. 

J.  On  the  Cultivation  of  the  Seyises. 

2.  On  the  Cultivfttioti  of  the  Meynoi*y, 

3.  On  the  TTse  of  Words* 

4.  On  Discipline. 

5.  On  Class  Teaching . 

The  Teacher. 

.■V  Monthly  Journal  devoted  to  the  interests  of  Teachers,  Schools, 
and  the  Cause  of  Education  in  general.  Subscription  price,  50 
cents  per  annum.  Specimen  copy  sent  free. 

Teachers  and  School  Officers  desiring  infonnation  relative  to 
oiir  publications  will  please  address 

ELDREDGE  & BROTHER, 

17  North  Seventh  Street, 

PHILADELPHIA. 

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